For initial building my motto has been 100 sites with $1 a day. Well actually it was 10 sites $3 a day. I have a site that rocks some days.. Another thing you can do is try to actually promote affiliate programs to offset your adsense income. Certain sites this should be more profitable than adsense, but others it probably wont be.
Best thing to do is get as many sites going as possible and when one takes off, work on it the most, then move onto the next site that starts to get successfull, meaning it makes more than usual. All the while your other sites are just getting older and older which is great in the eyes of google. I stopped looking at the day to day sales, it just makes you go nuts, it's great when the $XXX days happen but when you suddenly hit the low $XX day you'll think the sky is falling. Watch your averages, not your days. The reason it changes so much is because of the three evils, CTR, Traffic, Click price, sometimes all three are down, sometimes all three are up.
Ryos & Jellyfish - It's funny that you mention having many websites. I have been thinking long and hard about starting different websites but I am paralyzed by too much analysis.... Will I rank well in google? Is the ppc for my keywords too low? How to get Traffic, traffic, traffic?? The list goes on.... I have a subject that interests me but it is also a competitive field. I think I will try to find a niche within this subject.
I'm brand new at this, and I keep having ideas for new sites, so I keep adding more. Just go for it, certainly can't hurt anything. The worst that can happen is a site will fail miserably and you decide to abandon it. I've already abandoned a forum site. Seemed like way too much work to get it moving.
Every time I feel like I have stumbled onto the right combination for one of my sites, something happens to prove to me I cannot count on a particular monthly income. Combining A and YPN is helping me to set new goals, but I still hesitate to assume I could make a car payment every month. All I want to do is be able to buy my husband his dream car. Is that too much to ask?
I look at day to day earnings (or rather impressions) only on new sites - it's a good indication of when the traffic starts flowing to a site. For the main, I try to ignore daily variations on established sites - you have to or it will drive you nuts. I look more at weekly and month averages/trends. The nature of adsense is such that it is very very difficult to spot any problems within a site. You'd need to try and monitor the adwords costs for your keywords as well, even then you never really know what advertiser (that are showing on your site) are paying.