Fenders are an easy way to avoid this. To a lot of regular bike commuters a decent set of fenders is what separates a practical transportation bike from a recreational machine.
A slightly better option is a set of clip-on fenders. Clip-ons usually attach with straps or bungee cords of some sort to the seatpost or frame tubes. Some of them require a bracket to be attached to the bike permanently, to which allows quick installation or removal depending on whether you need fenders for that particular ride or not.
|A clip-on fender on the rear of my Fixed-Gear bike protects my saddlebag, saddle, and myself from road spray.|
Full-coverage fenders tend to cost a bit more, and take a bit of work to get in place, but once installed stay put and give better protection than most clip-on designs. Front fenders, especially, can be equipped with a mud-flap of rubber, leather or fabric that hangs nearly to the pavement, catching most of the front-wheel spray that would otherwise soak your shoes.
|My commuter bike's front fender, with a short mud flap that offers some protection, but doesn't really help my shoes much.|
|A fender with a full-length mud flap, which should keep the slush out of your sneakers nicely.|
There are a couple of potential drawbacks to full-coverage fenders. If you have to transport your bike in a car, and take the wheels off to make it fit, the fenders continue to take up room even with the wheels removed. Also, if you have a singlespeed or internally-geared bike, the rear fender might make changing a rear flat a bit trickier. Fenders that fit too close to the tire can get clogged up with mud or snow in the wrong conditions, and any fender can get a rock or stick jammed in it, but it's a rare occurrence. One common complaint with smaller bike in particular is that the front fender can extend far enough back that when the wheel is turned too far your toes might bang up against it while pedaling. Normally, you wouldn't be turning the wheel that far while riding and many cyclists ride with toe-overlap without a problem, but some find it extremely disconcerting.