I text quite a bit and I tend to have a lot of different groups that I message often (groups of specific coworkers, parents, friends) but not often enough that they stay on top of my 'recent recipients' list. I usually end up adding all the recipients to a brand new group chat and sometimes I forget to add someone.

I've started messaging a unique phrase in each group I have. So for my friends Kyle, Matt, and Johnson I type 'kmj.' Now that you can search message history in iOS 4, I just search for 'kmj' to bring up the previous group SMS.

If you don't want your friends to receive this message, send it in Airplane Mode. The message send will fail, just never bother resending it. It will still be searchable.

[crarko adds: I haven't tested this one. Seems simple, but practical, like Twitter tagging.]