If you have grass keep it very short, fill in your pond (i.e. anywhere frogs might want to live), don't leave rubbish/food out that might attract rats, and remove anything a snake might consider suitable for sheltering behind/in i.e drainpipes, old furniture, pieces of board etc. Oh, and buy a mongoose.

Definitely go with the mongoose, or I've found that the green power stuff(I think it's sulhpur?) works really well. Spread it around the edges of the garden/house and it should keep them away.