I haven't heard of any snake repellent that works very well, or works long term (has to be reapplied monthly or after every rain, etc.) The best way to keep them away is to remove any suitable habitat from your area of concern. Any kind of piles (rock piles, wood piles, debri piles, etc.) is a perfect hiding place for snakes. The don't like wide open areas, so if it is your yard you are refering to, keep grass mowed very short and use a weed-eater on all fences lines and stuff, and also remove any piles or cluttered areas. If you do find a repellent that works well, just keep in mind that it will also keep snakes trapped in the area! The only way I can think of to truely keep them out would be to build a secure privacy fence around the area with absolutely no gaps or holes anywheres AT ALL for them to get through (I assume your in North America, where our venomous snakes are terrestial, and do not climb) but I think such a fence would be impossible to do. If you do find something that works and it's not a response here, please post it so we know about it!
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-David
Check out my pet pics at:
http://www.myspace.com/obx_fisherman
1.0 ball python (Pandora - don't ask)
1.0 argentine boa (Prometheus)
0.1 hogg island boa (Andromeda)
0.0.1 brazilian rainbow boa (Inara)
1.0 kenyan sand boa (Diablo)
1.0 normal corn snake(Cypress)
0.1 amery. corn snake (Morgan LaFay) RIP 
0.0.2 baby corns (Romulus and Remus)
- 1 normal, 1 ghost
0.0.1 banded cali. kingsnake (Cain)
1.0 tangerine honduran milksnake (Narcissus)
0.0.1 snow corn snake (Valkyrie)
1.0 amazon tree boa (Pegasus)
1.0 colombian boa (Poseidon)
0.1 albino san diego gopher snake (Athena)
0.0.1 sandfish skink (Slick)
0.0.1 fire skink (Phoenix)
1.0 dog (Luke)
the wife's pets:
1.0 bearded dragon (Leonidas)
1.1 ferrets (Ares, Enyo)
1.2 cats (Galahad, Ripley and Sassy)
0.1 Boxer (Zoe)
NEWEST ADDITIONS:
1.0 adult Dumeril's boa
0.0.1 baby yellow amazon tree boa