This has been discussed a number of times, and deserves to be revisited periodically. The most common suggestion for the frequency of testing on the hobbyist side of the equation has been every six months, as I recall.

STD testing is relatively easy to do. The first visit is the toughest, since you're venturing into uncharted territory. Most towns of any size have a Planned Parenthood center, who will do the testing for you. They tend to have a number of questions about your sexual activities on the forms you fill out, but I have never found them to be judgemental about the answers I give them.

As Wild Child suggests, put this in your calendar and into your hobbying budget.

Other options are pull out your yellow pages and look under clinics.  There you will find places that will mention "discreet" or "confidential" testing.  They may cost a bit more, but when they turn your information in to labs, most times they don't even use your real name and will not turn your name over to the state if you to have something minor, whereas, as a warning Planned Parenthood in certain states does.
I have been fortunate because I have found a doctor to go to who knows what I do on the side and every other visit we actually turn it in to my insurance company to cover costs for having a "cold".  Sneaky on his part, yes.  However knowing there are people like that out there, makes the testing all the more easier.  If every clinic/doctor was as discreet, more people would take the time to get tested and catch things earlier and perhaps just maybe this whole STD/STI thing would go down.
Scary to think that Herpes is now becoming accepted as a norm almost for anyone who has sex.  Unfortunately it's so common, that they have commercials for it during prime time TV
Just a friendly reminder to be responsible, to yourself and others.  Getting off the soapbox