Since the most recent release of piston 0.3.3, you can use Steem's swiss army knife for multisig transactions. This tutorial gives a brief introduction and shows how it works.
But first, let me clarify a few technical terms:

What's a multisig transaction

The term multisig refers to the requirement of having more than one signature to create a valid transaction. Most transactions don't have this requirement because they are tied to regular accounts. However, you can extend your account to a multi-authority account by adding more keys and requiring more than one of them to sign a transaction.
In simple terms, you could secure your funds in such a way that you have one access key stored on your computer and another stored on your mobile phone. Then you can setup a scheme where you can only transfer funds when both, your computer and your mobile phone sign the transaction.

What are "multi-authority" accounts

A multi-authority account does not consist of just a single access key (e.g. password) but can consists of one or many

public keys (read: passwords) or
other account names.

This allows for some very cool setups for improved security and trust. Let's say you have 2 friends and you want to setup a company together with them.
Then you create a new account remove the initial public keys and put their account names into the authorities. Now you have an account that is jointly owned by a group of people.

What are authorites and permissions?

On Steem, there are 3 permissions:

active: the active permission can move funds and trade in the internal exchange as well as change all permission authorities (except for the owner permission)
posting: the posting permission is required for posting and voting on Steem
owner: the owner permission is the super administrator and can change and overwrite all other permissions

All permissions have a threshold that needs to be reached by signatures. Regular accounts have a threshold of one, such that only one access key (password) is sufficient to access the account's permission.

Each permission can consist of one or multiple authorities and a weight
Two choices exist for the authorities:

a public key
an account name

and the weight is an integer number. The weight, together with the threshold of the permission, works like this: If the sum of the weights that are associated with the signatures exceed the threshold, the transaction is valid.

For example: Let's say we take a look at the active permission of account xeroc. We can take a look at its current permissions using piston:

$ piston permissions xeroc
+------------+-----------+-----------------------------------------------------------+
| Permission | Threshold |                                               Key/Account |
+------------+-----------+-----------------------------------------------------------+
|      owner |         2 |                                                fabian (1) |
|            |           | STM7mgtsF5XPU9tokFpEz2zN9sQ89oAcRfcaSkZLsiqfWMtRDNKkc (1) |
+------------+-----------+-----------------------------------------------------------+
|     active |         2 |                                                fabian (1) |
|            |           | STM6quoHiVnmiDEXyz4fAsrNd28G6q7qBCitWbZGo4pTfQn8SwkzD (1) |
+------------+-----------+-----------------------------------------------------------+
|    posting |         1 |                                             streemian (1) |
|            |           | STM6xpuUdyoRkRJ1GQmrHeNiVC3KGadjrBayo25HaTyBxBCQNwG3j (1) |
|            |           | STM8aJtoKdTsrRrWg3PB9XsbsCgZbVeDhQS3VUM1jkcXfVSjbv4T8 (1) |
+------------+-----------+-----------------------------------------------------------+

We see that the threshold is 2 and there is one key and one account with each having weight 1. This means that we require a signature from STM6quoHiVnmiDEXyz4fAsrNd28G6q7qBCitWbZGo4pTfQn8SwkzD and from the active key of fabian. to construct a valid transaction that spends from account xeroc. And we will see below how that will work

Setting up a multisig account

So, in order to do multisig transactions, we first need a multi-authority/multi-sig account. We can use piston to set this up.

Remark: Since we are changing permissions of accounts here, I highly recommend to use this with a temporary account first and also to use the -dx flag (nobroadcast) to verify the transaction before broadcasting it.

Recommendation: We highly recommend to first test with the posting or active permission and only if you feel comfortable, change the owner permissions.

Adding an authority

We can add a named account or a public key using piston allow. We need to define the affected account as well as the permission to modify.

piston -dx allow --account xeroc --permission active fabian --weight 1
piston -dx allow --account xeroc --permission active STM6quoHiVnmiDEXyz4fAsrNd28G6q7qBCitWbZGo4pTfQn8SwkzD --weight 1

These transaction will only add another authority to the permission with the provided weight.

Adding an authority and changing the threshold

To change the threshold of your account, you need to use the --threshold x parameter with your last additional authority.

piston -dx allow --account xeroc --permission active dantheman --weight 1 --threshold 2

This will change the threshold.

Verify the multisig account

Using piston permissions we can take a look at the end result of our actions:

piston permissions <accountname>

Spending funds from a multisig account

Spending funds from a multisig account is as easy as using piston for coldstorage. The major difference is that you need to transfer the partially signed transaction between multiple parties.

Note: Due to the limitation of the expiration time to a maximum of 1h, you will need to find a time when you reach everyone within the hour.

Create an unsigned transaction

Let's create an unsigned transaction using

piston -x transfer --account xeroc fabian 0.001 SBD > unsigned-transaction.json

Send the unsigned-transaction.json file to all relevant parties and let them sign the transaction

Signing unsigned/partially signed transactions

The unsigned/partially signed transaction can be signed with the available keys using

piston sign --file unsigned-transaction.json

The result can be safely send to the initiator or be broadcasted if all required transaction have been added.

Signing Party

Technically, it's your decision on how to collec the signatures.
Either you let them all append their signatures and forward the improved partially signed transaction (assuming they know each others contact data), or
you let them send all the signed transactions back to you (if only you know who has the keys). In the latter case you would need to copy/paste the
signatures into the transactions so that it takes the form below and can then broadcast it.

{'expiration': '2016-09-07T09:16:22',
 'extensions': [],
 'operations': [['transfer',
                 {'amount': '0.001 SBD',
                  'from': 'xeroc',
                  'memo': '',
                  'to': 'fabian'}]],
 'ref_block_num': 39520,
 'ref_block_prefix': 4016647731,
 'signatures': ['1f52fe34142a421ff711f0ddf29b0f782b74b68d9330380b464f44dbf59ab291b208f9969ec4bd215570b796e4f036d1a5ab37b84cdf2d9ad4d36162a799ebcd8f',
                '1f1037cfe13b1f278fb2cae6b588dcd6a7d24de7ca26c29d1a7a70c4646b39d0d21c35749f444fb5b2686fd8552fe89b9013ab5723f1f4c5ba394c6e1a92ffb489',
                '1f67430dd482848d14cfce7c5de11628b0cbea3cf3b0ced546b64172abb730cfed797da9490c66b2208d24d9ea24654b47e9ce758aa6f19b4bbb0dbd1cc1afe41c',
                '20270dbcc95af22cc55404ff5b220a8aaf2585c3f47d496af1ae426c7d68f9e5c471d8dbc98c728bbeeec09dc8a47ddb58f3f55e67f5b603fdfd1ead47e8ffcb6a']}

Broadcasting the signed transactions

Once the signatures are put together into one transactions, we can broadcast the transaction using

piston broadcast --file signed-transaction.json

The operation should (if the signatures are sufficient and valid) be executed within seconds.

[piston] How to use it for multisignature accounts