Dow 30 is a good list; liquid. Similarly any other liquid list is good.
But..
you are exposed to close to 1 correlation. In long stretches of time (months) when markets go one way or another your list will all look like 1 stock instead of 30 different ones.
Another idea is to keep another list of diversified etfs. Liquid ofcourse. Say all the i-share series or pick a few from each sector/market groups. Add in few tickers from completely different asset groups (think.. gld, slv, bonds).
I posted a list a while ago on ET of a somewhat diversified list i kindof trade and watch. But you should be able to get more ideas just doing a general search for diversified portfolios.. or use keywords like "permanent portfolio" etc.
Good luck!
-gariki