The best way is to follow NH-47
Bangalore-->Hosur-->Salem-->Coimbatore-->Palakkad-->Trichur-->Cochin
-->Allepey(Alapuzha)-->Kollam(Quilon)-->Trivandrum
As easy as that. And while you are in Allepey take some time off to get into a houseboat and check out the backwaters.
Malludude's given you the perfect route for you to take. You would nt get bored driving that route. Did exactly that route. JKdas posted the perfect link for you to get all the info you need.
I did a trip to Trvndrm last month and enjoyed the route. These guys give you just the right info. you would need.