Suprised I haven't read any responses (I did only scan the posts quickly) that mention peeing after having sex... It's a non-medical form of profylaxis, but one that can help you out a great deal as well. It's possibly tricky and annoying after orgasm but it does help preventing the bacteria to move upwards into the urinary tract and "sticking" to the bladder epithelia.
On a sidenote, bacterial resistance to drugs is more likely to develop at low doses rather than high doses. Bacteria have a high mutation frequency. Most of the times, if the mutant isn't better than the original bacteria, it will not have the chance to replicate (enough) and the original bacteria will "swamp" it. At the low doses, the bacteria with a little higher "resistance" to an antibiotic, would have the chance to grow more than other bacteria that are more susceptible to the drug. That's how resistant populations occur. Now when the antibiotic dose is high enough, the bacteria that got "a little resistant" (either through a change in enzymes or cell wall structure or...) will still be killed by the antibiotic because their mechanisms against the drug are overruled. That's why the current guidelines in antibiotics are aiming at using high enough dosage forms.
Errr... well in case anyone was interested in the details ^^