How do you play the trill in measure 29 of Beethoven's moonlight sonata?
I would play it like this (the image came out kind of big...):
You can play the lower A# quite short to avoid tension (if it suits your interpretation, of course). For me the most natural fingering is 4-5-4-3-4-5 in the upper voice, although 3-5-3-2-3 works, too, and might be easier if your hand is big and/or 4th finger weak.
There's a similar trill a couple bars later, which I find much harder. The fingering 4-5-4-3-4-5 works there, too, or you may try 5-3-5-3-5-4 or something similar. In both cases it helps to play the first note of the trill a bit longer and stronger than the others, and it works musically, too.
Beethoven always started his trills on the upper note (documented here), unlike modern practice. This would come out more fluent coming onto the G#. Although Nonpop's example could work, it's not normal to start and end on the same note. A better interpretation would be
B A# B A# G# A#. As far as fingering, without context, it is hard to say what would work best, but 4-3-4-3-2-3 should work. As far as timing, whatever goes with your interpretation is fine. I've shown two alternate timings below.