Some build up on the glass will happen with burning, especially when you have a slow burning fire.  Excessive soot or creosote build up on you glass can be caused by a few things:

1. Excessive moisture in you wood: Make sure your wood has no more than 15% moisture

2. Not waiting long enough before shutting the stove down: Burn it hot for at least 30 minutes with a bright crackling fire.

3. Using too large or unsplit wood pieces that just smolder: Split your wood smaller and arrange it so that there is plenty of airspace between the pieces.

4. Loading the wood too close to the glass: Leave 1" of space between the glass and the wood for proper air-flow

If you glass gets dirty, you can try burning it clean with a hot fire. If you can clean the glass with a nice hot fire, then you will know the technique to keep your glass from getting dirty. If the creosote is too thick, you may need to clean the glass with a glass cleaner designed for wood stoves.