My app sales track record
From what I saw with my first App, Time Task Calc, I had an initial peak that lasted 3 days, then it dropped off to about 1/5 the peak, and dribbled along at that pace for a while. I experienced another peak when I released my bug-fix update, but it was only half of what my original peak was. I've sold at least 1 of TTC every day, but some days that's all I sold.
My new App, Informant, has a wider target demographic, and it has been VERY visible on my app sales. My first day of sales of Informant was more than my first week of sales for TTC. I'm on the third day, and it has begun to drop off already. Strangely I don't have even 1 review yet, so that might be hurting me.
I keep a close eye on my 'popularity' on the iTunes Store as a sort of gauge, but it isn't terribly revealing.
Each app seems to fare differently, depending on a bunch of factors.