I'm finding I'm not getting good carbonation at the tap.

I'm putting the beer into a 5 gal keg with a mixture of about 3 1/2 oz. of priming sugar and 8-10 oz. of water as directed and letting it set @68 degrees for 10 days.

I have since read somewhere(?) that it is a good idea to vent the O2 out with CO2.

I have to leave it on tap(@10 lbs) for an additional week and a half to get a good clean head. I can't wait, want beer now.
-the first couple of beers are okay~~~~~, but it immediately goes flat on 3 and 4.

Is adding the CO2 a bad idea? Should I use more sugar?

PS. Stihler, I'm doing my honey wheat with honey/carbonation mix now. I'm so excited, I'll let you know.

If your using priming sugar, I would use 4 ounces and let it go for at least 14 days... When I keg my beer I crank up the co2 to 28 psi for 2 days and then back it down to about 9 psi or so.. The beer always comes out just the way I like it.. good luck!

So you leave it at 10 psi while serving? It should stay the same from the first beer to the last once it is fully carbonated. Venting the o2 is a good idea and it only needs about 5 seconds or so to push it out with the co2 hooked to it. Then you can disconnect the co2 and let carbonate if using priming sugar.
I do not use any priming sugar. I have a quick writeup on force carbing on my website. I have used this method for a couple years and i can have it carbonated in minutes.
Check out this link!!
http://flatfenderbrewing.com/howtokeg.html