Not all glass door fireplaces are built alike, but there are a few tips that might make the job easier no matter which style of door you have. The first thing to remember is that it's always tougher to clean your doors once the soot is really baked on. This means that regular cleanings are a key to making the job a little easier. In the winter, try to clean your doors every couple of weeks for best results.
Another tip is to remove the glass before you clean it. Some doors have simple metal clips that slide over to allow removal of the doors -- just be careful not to break the glass. Other doors may have screws that make removing the doors a little more complicated. It's up to you whether or not it's worth it to take them off. The one thing that removing the doors will do is make it simpler to clean in all the nooks without getting pasty cleaner on the frame.
You should also be sure you have the required equipment to clean the doors correctly and safely. You might want to wear rubber gloves when using the glass cleaner, so keep them on hand, along with some dry cloths or clean paper towels. It's also necessary to wait to clean the doors until after they've completely cooled down. Handling warm or hot glass is dangerous, and the cleaner is most effective on cool glass.
If you find that you've taken a little too long between cleanings and the soot has baked on there pretty well, try using a razor blade to scrape the stubborn bits of carbon off before you use the glass cleaner. And finally, if you seem to have a problem with regular cleanings, you can always go with tinted fireplace door glass to help keep the soot from standing out so much.