Leaking forks is one of those things that plague off-road riders. Many think that if your forks are leaking then it's time to replace the seals, but that is not always the case. Before I go any further, however, I need to put in a disclaimer here. If your forks have been leaking for a while and you have been ignoring it, or it's been a while (maybe never) since the oil has been changed, then you should have the forks gone through no matter what. Fluid loss can lead to unstable handling, and worse, damage to the internal workings of your forks. Also, over time as things wear, your forks build up grime inside so they need to be cleaned and maybe have wear parts replaced. Don't ever attempt to clean a leaking shock seal using this method, shocks are charged with high pressure nitrogen and should only be serviced by someone that knows what they are doing.
Now, having said all that, back to what I was really writing about. If your forks have just started leaking, don't assume it's time for seals. More often than not, a leaking seal is caused by very small bits of debris getting caught between the seal and the fork leg. With a little patience the debris can usually be cleaned and you are good to go. So the burning question in your mind right now is "how do I clean the seals?". Okay, maybe that is not the burning question, but if you are still reading you are least mildly interested, so here you go.
The first thing you want to do is make sure the outside of your forks are clean. Using a brush, water and some liquid dish soap carefully clean all the dirt, oil and other grime from the the fork legs paying particular attention to the wipers (the part that looks like a seal that you can see from the outside). You should probably remove the fork guards before you wash just so you can really get in there and clean all the way around. Once you have the forks clean, put the bike on a stand and get something to sit on. The key to this is taking your time, so if you are squatting and uncomfortable you will most likely want to rush through the process. Using compressed air, blow away any remaining dust from the wiper and fork, then with a flat blade screwdriver, carefully pry the wiper down working your way around the upper fork tube, remember to go around the tube and work it down instead of trying to pry it completely away from one side. Once you have the wiper out of the way you have access to the actual seal. You may be able to see the spring clip, it might be tempting, but don't remove the spring clip! Using non-chlorinated brake cleaner (you can use chlorinated but it stinks and doesn't evaporate as quickly) liberally spray the seal area to wash out any dirt or grime that might be hiding up in there; following with compressed air to help blow out debris and evaporate the brake cleaner. Don't go crazy with the compressed air, you don't want to push dirt up through the seal and into the fork oil.
Now that you have the area clean it's time to clean the seal itself. Remember, leaking is normally caused by dirt getting between the seal and fork leg. The last thing you want to do is introduce more dirt, or push the dirt up inside the fork. I have heard of a variety of "tools" used for the next part; feeler gauges, business cards and even wrapping the area in black tape and compressing the forks (essentially pushing the dirt into the fork). Personally I don't like any of those for a variety of reasons. I prefer to use either a piece of plastic like you find on product packing or even better, Motion Pro makes a little plastic tool that works really well. Using your tool, carefully slide it between the fork leg and the seal. With a little sawing motion, work the tool around the leg. I like to start off doing about 1/8- 1/4 of the way around the leg, pull the tool out and wipe it off. If you look carefully you will probably see very small dust particles in the oil on the tool. Wipe your tool with a clean paper towel or rag and keep repeating this process until your tool comes out with just clean oil and no particles. This is the patience part, you may have to go around several times before you get it clean. Relax, have an adult beverage if that helps. Once you have cleaned the debris, wash down the outside again with brake cleaner and compressed air. Remember to clean the inside of the wiper as well. Put the wiper back into place, take the bike off the stand and compress the forks several times. I like to clean the fork legs with a clean towel and brake cleaner after each compression. If the leak stopped you are good to go, but if one or both of the seals continues to leak, either you didn't get them clean or it's time to replace the seals and have the forks serviced.
Keep in mind that if you ignore the problem it will only get worse over time. Also take a good look at your front brakes, if you pads have been fouled with oil they should be replaced. Once you have done all that and you have metabolized your adult beverage get out there and ride the thing!