Use more descriptive words, and be careful not to let it get stale. Familiarity is a big one. The characters will say things that become speech patterns. Readers will come to have expectations about their routines, the places they are, the way they'll react in a certain situation, what they might say to someone, etc. In a shorter piece, there is not enough time to develop these little characteristics.
Dialogue is another tricky bit.
It needs to be much more complex than in shorter pieces. Shorter pieces actually don't need any dialogue, but novels do. Remember the general rules for dialogue in the article on writing good dialogue. Try to make it as realistic as possible. You'll finish them and be proud of your work.