Taking bum's post a bit further, why don't you buy a 6 pack of beer from the shop, and pour it in a glass and then do the same with your homebrew and see if there's a difference in head retention between the commercial stuff and your HB? That way you have the same pouring style, rather than comparing against someone else's pour and you can trial them side by side.
I had the same question a while ago and I did the same thing and realised that it only seemed like the head disappeared quicker on the HB when, in fact, the head was retained for approximately the same time as the commercial beer.
The wheat malt does do a good job for head retention, but costs a bit more than standard malt. I found that just buying malt extract to use instead of those brew boosters also seemed to help.
Is your beer actually flat? Ie, when you take a swig, does it feel like flat Coke does? Or does it still have carbonation then? I found that while it may not be bubbling in the glass, when you take a swig, it's still carbonated (not like a soft drink though) but it doesn't appear to be fizzy in the glass. I dunno, that's just what I found when I started out. The other solution if your beer is going flat quicker is kinda obvious.... just drink it quicker! :chug:
Cheers.