What I've been meaning to do (although I haven't yet... misinterpretation of the virtue of Laziness) is mix the skills in a project. For instance, to keep my Perl up to date and improve my Unix and DB skills, write a script to parse syslog, insert entries in an appropriate database, and generate reports. (The syslog parser would run as an hourly cron job, the report generator as a daily one.) This keeps me practicing my Perl and DBI skills, gets me more into schema design, and involves me in such things as syslogd(8), ipfw(8), and so on. If I decide to make pretty-printable reports, this project could get me into PostScript, and so on.

So the basic idea, for me at least, is to work on things that involve a broad range of skills. This is a bit difficult to do at work, since I usually get handed the Perl part of a given problem, but for personal projects it tends to be pretty easy. I haven't run into too many problems that are particularly narrow in their focus.