First thoughts are make sure you've got decent gear - if you're cold and wet it'll sap concentration levels and lead to poor decisions which is never good.
Lack of vision (rain / traffic spray on your visor) can be more of a problem than lack of grip sometimes - in which case, obviously slow down. What sometimes helps here is moving your head slightly (if you have a fairing, duck your head slightly) to use the windblast to your advantage (it'll depend on the exact conditions and speed), but the wind can help clear water off your visor and aid vision. It may be awkward on your back though.
As stated, make sure you're bike is fit for it, especially as regards tyres / suspension.
If you find it hard work keeping concentration up in poor weather, consider taking more frequent rest breaks (if you're commuting, it may mean leaving for work earlier) - better to take a bit longer and arrive in one piece than to not arrive at all...I got caught out in snow in May (who the hell predicted that??), and I went a few miles (until the cold kicked in), stopped for a cig by the side of the road, and carried on another few miles. It were a long run, but I made it in one piece eventually.
Beware of standing water, and beware of the muppets that hit it at speed: if you're on familiar roads, you'll know the likely risk spots for it, obviously slow down and take it steady through, and beware because cagers *will* insist on hitting pools of standing water at speed and drenching you (I've had it happen to me - not fun).
Similarly, beware of roads with lots of trees etc - if its been raining and the sun (sun, wtf is that??) comes out the open parts of the road may dry off, but shaded parts may still be wet where the sun can't get through ("micro-climates", as they're termed). Same rule applies in winter where it may be icy.
As well as slowing down, look further ahead, which will in turn encourage you to be smoother with your bikes inputs - anticipating a situation and adjusting your speed accordingly is better than carrying on braking late or snatching a gearchange because you weren't ready for it.
Finally - stick to your own limits and take it easy.