Mulching your grass clippings is a great way to provide extra moisture and nourishment to your lawn. It also represents a significant time savings in comparison to bagging or using a side discharge system, which of course requires raking (click here for more time saving tips). Unfortunately from time to time large clumps of grass will be left on top of your lawn as you mow. These clumps of grass look bad and will block out the sun and temporarily kill your grass in that spot. Needless to say, clumping should be avoided as much as possible. If you do find clumps they should be cleaned from your lawn as soon as you can. I can't tell you how many times I have walked around my almost 1 ache of lawn with a bucket collecting clumps. I have even recruited my wife and daughter to help. Sometimes clumps are unavoidable, but there are a few things you can do that will help reduce or eliminate grass clumps as a result of mulching. So with out further ado, here are my tips for avoiding grass clumps:

1. Never mow your lawn when it is wet. Never mind the fact that its not safe, mowing when the grass is wet is a sure way to get clumps all over your lawn. The grass will stick to your mower deck, your blades, and anything else it hits. Your deck will become so clogged that it will struggle to spin. Big blobs of grass are sure to litter your lawn if you mow before the lawn dries.
2. Clean your deck. Eventually the underside of your deck will likely build up a quantity of grass that reduces the space needed for proper mulching. Be sure to clean your deck regularly, and don't forget to follow proper safety procedures so you don't get hurt.
3. Raise the Cutting Deck. If you are trying to cut too much grass at once, you will likely end up with some clumping. Raise the deck and make a first pass. Then lower your deck to your desired finished height for a second pass.
4. Overlap your previous cutting paths: One of the main culprits causing clumping is too much grass being cut at once. To properly mulch your clippings your mower needs to cut each blade several times to get the clippings nice and small. To do this, your mower needs room to work. If you are cutting too much at once, your mower will fill quickly leaving little room for additional cuts to take place. Overlapping the path your took on your last pass will allow for a bit more room under the deck. You will need to experiment to find the amount of overlap that works best for your situation.
5. Sharpen your blades. Dull blades can't cut as well as sharp blades. Instead of mincing your grass clippings up in to tiny, beneficial bits, dull blades are unable to cut sufficiently to do the job. Take your blades to have them sharpened on a regular basis, or get a sharpening kit/ file and sharpen your blades yourself.

So you followed all of my advice and you still get tons of grass clumps all over your yard. If you are looking to have the best looking lawn in the neighborhood, you have no choice you have to clean them up ASAP. I have had some success using my sweeper to pickup some of the clumps. Of course this will only reduce the number of clumps that you have to pickup by hand. Sweepers don't get the really heavy clumps especially if you have run over them with your mower tires.