Practice inventing motifs of 3-4 notes. Work toward the ability to transpose them (and play in sequence) fluently diatonically and chromatically.
Learn to transform short motifs in a variety of ways (augmentation, diminution, backwards, inverted, change rhythms, change mode (e.g. major to minor) and so on).
Use short thematic material or motifs that you (or your audience) might like to (and be able to) whistle.
Lower your expectations. Go for quantity. Holding out for perfection is a great way to strangle your creativity. Start with something (anything) and then make something out of it.