1. Unstable tonal plan. Usually the developments of the sonatas don't have one key which could function as the main.
2. Avoid the tonic key. If you use the tonic key extensively in this section, there will be no drama and no conflict. It's best to use other keys.
3. Use mostly the keys of subdominant area. Since the Exposition of the sonata modulates very quickly to the dominant area and the Recapitulation returns to the home key, the Development is very much suited for the keys of the subdominant area (IV, VI, II etc.)
4. Shorten the theme into fragments. A very useful feature in many sonatas is that the theme is presented in smaller fragments - phrases and motives.
5. Invert the theme. To further complicate things, change the direction of the intervals in the theme - ascending intervals become descending and vice versa.
6. Change the intervals of the theme. When your theme has characteristic intervals, such as leaps, you can widen them or make the narrower depending on your choice.
7. Use augmentation. The rhythms of the theme can be doubled - instead of eighth notes write quarter notes, instead of quarter notes write half notes etc.
8. Use diminution. In some cases, you can even use rhythms in the theme that are twice as small.
9. Use sequences. Sequences are wonderful for modulating into foreign keys. Choose any interval for sequencing, such as major or minor seconds, major or minor thirds, perfect fourths or fifths.
10. Use imitations. Imitations are perfect for creating dialogues between the parts. Choose a motive from one theme and write it in various parts from various pitch levels.
11. Employ counterpoint. Don't forget to write independent melodies with the help of invertible two or three part counterpoint. When done correctly, this technique will work wonders on the development of your sonata.
12. Sonata has 3 sections. Introduction, the Main section and the Dominant pedal point.
13. Introduction. In this section you can show the main theme or the closing theme in secondary keys.
14. The Main Section. This section is the longest - here you can develop the motives of one or several themes in many ways.
15. Dominant Pedal Point. The drama of the development ends with strong emphasis on the dominant key area which serves as a preparation for the Recapitulation.
16. False Recapitulation. Sometimes the Recapitulation is postponed by the entry of the main theme in the secondary key, sort of "by accident" after which the theme in the main key immediately appears.
If you want to compose or improvise a sonata, you may also find this article helpful to read.