March in like you own the place, get in a stall, do your thing, wash your hands and leave. Don't fuss over yourself in the mirror. Don't eyeball anybody, don't make conversation.
If the toilet has urine all over you have five options.
1. If you have some method at hand of standing to pee, obviously, there's that.
2. If no STP method available, grab a preposterous amount of TP and wipe the seat clean.
3. Check a different stall. Disadvantages: may attract attention doing this, and chances are the others aren't better anyway.
4. Highly unrecommended and unsanitary, but I suppose it's possible for one to just go ahead and sit in some other dude's urine.
5. Leave without using a toilet. May not be feasible depending on how bad you need to go and how long it'll be before there's another option. Holding it for too long can contribute to urinary tract health problems (infections and worse), so try to avoid doing that more than necessary.
If all the stalls are taken, personally I prefer to leave and come back later rather than hang around, seems kinda weird to just hang out in the bathroom.
And finally, if you have a method of STP and are comfortable using urinals, bear in mind the rules of urinal usage. Google "urinal etiquette" or "men's bathroom etiquette" and you'll get plenty of pages on the subject, some more serious than others.