My forgotten sites often do better in the serps and make more money than sites I put shit tons of time and money into. I've been in serious 'build mode' the last three years. I've built way too many sites to actively update and monitor. It's worth keeping them though because aged sites/aged links kick ass.
I had one site that I haven't touched in over a year, it's now topping the serps. I didn't know this until I got an email from a potential buyer. I called the guy and he offered me $5000 for the site. I turned down his offer thinking I can get more in the long run if I keep it. I've been working my ass off on the site and spending tons of time on the phone trying to sell advertising and leads. I've got it making almost $300/month now. Chump change considering the hours I spent in the last month or so. I know that once I give up and start investing in other projects that advertisers will be contacting me. Time to forget this site again.
Honestly, the less I work on a project, the better it seems to produce. Build, tweak, promote, give up, and forget.....then bank.