Level size -- start from the top left and make the level bigger than what you expect you'll need. If it turns out you have extra space, you can resize the level, and hopefully, you won't lose any tiles (unless you REALLY wasted space).

Tileset -- Make um....um... The Underground -- a tileset with PROPER cave scenery and stuffs.

Listen to music and set your work in increments. Agree with yourself that you'll work from point A to B and once that part is done, you do something really enjoyable. You'll work faster, and quite possibly, get more work done than you expected.