You need the suede cleaning block that forever skint mentioned. Clarks call it a 'nubuck block'. Either that or a 'crepe brush'. Suede brushes are too harsh as they are wire and like Poppy says remove the suede. A crepe brush is made of crepe (suprisingly!) and as you rub the suede or nubuck the crepe gets hot and sticky and dirt and stuff sticks to it.
A nubuck block costs £2.95 & the crepe brush is £3.20. This is from Clarks although I imagine a supermarket or local discount store would sell their own versions a lot cheaper. I always find the nubuck block works better - its like an eraser and rubs of any marks although very stubborn marks may not come out.
Once you have cleaned them then spray them with protector spray. As well as making your boot water resistant it will also make them stain resistant so they will be easier to clean the next time. OMG - listen to me, I'm like a walking advertisment - can you tell I work in a shoe shop
Sometimes it's important to work for that pot of gold...But other times it's essential to take time off and to make sure that your most important decision in the day simply consists of choosing which color to slide down on the rainbow...