Please do not put arbitrary data in OP_RETURN outputs. This is antisocial because you are forcing everybody using the network to indefinitely store (or at least receive) your data, which presumably is (a) relevant only to you and your recipient, and (b) only revelant at the time of the transaction.
How to do this properly depends on your goal:
For
timestamping data, I think in the other thread some people mentioned services that bundle up many messages and commit to them all in a single OP_RETURN output. This is much better and (IMHO, as only one participant who is affected by the extra data) perfectly fine. It winds up costing everyone 40 bytes a day or something on this magnitude, regardless of the number of users.
To
attach a message to specific transaction, send it out of band and use pay-to-contract to commit to it. This costs zero blockchain space and ensures that your message cannot be changed or corrupted. Unfortunately I'm not aware of any wallet support but the math is very easy (described in
Appendix A of the sidechains whitepaper), and is done in
contracthashtool and
pacthash for use in the sidechain Elements Alpha.
Note that in either case you need to send the message out-of-band, and are only using the blockchain for commitment, timestamping, and associating it to a specific transaction.
