Surface accessibility routines always work on the rigid coordinates. An atom that is very close to the surface can often be called buried, although it will probably be accessible, and therefore most likely hydrogen bonded to bulk-water most of the time due to thermal motion of some atoms nearby. Filling cavities in a protein only works in theory. In practice it does not bring much. There are many experiments where people introduce cavities and show that you could in principle gain about one kilo Joule per Mole per buried carbon atom.
Entropic stabilization is described above.
This is especially important for proline. Helix capping is your other sure bet. The biggest problem with cysteine bridges is that if you make only a small mistake in the geometry, the thing will cost you already lots of energy. I suggest that you only try cysteine bridges if you can find them in an homologous molecule, or if you are fighting with irreversible denaturation. If, however, you want to reduce reversible unfolding by putting in a cysteine bridge, then that bridge will work better when the connected positions are far away from each other in the sequence.