The strategy I use for my hair style (basic flip up in the front) is use the Toppik first without any hair products, then pat it down to make sure the fibers are distributed evenly. For the front I use the hairline optimizer offered on the site to good effect so when people look at you straight on it looks thick but not black up front and obvious.
After applying Toppik, I use a small amount of Crew grooming cream just to flip up the front. At this point I can see if any Toppik has fallen on my forehead right below the hairline and get remove it. I always make a sweep with my index finger along the hairline to get away any of this.
For the top, I make sure the Toppik applied on top has covered all the thinning areas and then apply a hair spray to hold the fibers in more securely. I use the one they offer on the site but I've read other hairsprays are just as effective. For the basic flip up style that is so popular still, this is the best method, and looks very authentic (my g/f is still fooled). Also, if you're using Rogaine, make sure the area has ample time to dry/soak in the minoxidil before applying Toppik.
If you insist upon combing the top of your head, follow the steps as listed above, e/c before you apply the hair spray, style it using a plastic-bristled comb (NOT a brush), then check to see if you've opened up any gaps in your Toppik coverage, and finally apply the hair spray. Unfortunately, it's pretty hard to pull of the "wet" look (if you liked to use a lot of gel, etc.) effectively with Toppik, but I think you'll be happy with the results above, especially if your hair loss is moderate recession, moderate diffusion, and vertex thinning like me.
Hope this helps!