Modular Synth Panels

Here's the main thing I'd like advice on with soldering. I've ruined a
few tips so must not be tinning/cleaning correctly. I have a little
container of the Radio Shack tip tinner. What I do is fire up my iron,
dip it in the tinner, wipe the excess off on a damp sponge, solder,
wipe on damp sponge and then dip in tinner. It then sits for a few
seconds while I get the next component ready, I then wipe on damp
sponge, solder, repeat... Eventually parts of my tips have started to
turn black and won't heat up so I have to replace them. 2 things I'm
wondering about are when I'm done soldering I wipe on sponge, tin it
pretty well, then turn the iron off. Should I not tin the tip before
turning off for the night or whatever? Also, the stuff in the tinner
container starts to look pretty nasty, and the way I use it is to
submerge the tip and get it pretty heavily coated - is this correct?
Are you more supposed to wipe along the surface, as you would with a
sponge, and not use the sponge at all?