If your grass is wet, don't mow it. If you mow wet grass you will not get a clean cut. Wet grass tend to lean over more, and it is also more pliable so it is more likely that the blade will push the grass over instead of cutting it. What this all adds up to is a poor cut where some of the grass doesn't get cut to the same height (or at all) so when the grass dries out it is very uneven. Cutting wet grass will also lead to grass clumping up in your bag and sticking to your blade, further reducing the effectiveness of your cutting.

If you absolutely must cut wet grass, spray the mower blades with vegetable oil first (with the mower off of course). This will help reduce the amount of grass sticking to the blade. You'll still get an uneven cut, but it won't be quite as bad.