I always say "Read the books" - so as said, do a lot of homework first, so you know as much as possible about each year and the differences/peculiarities of each. Look long and hard at your potential purchase. Use the CCCUK Buyers Guide. Buy the best you can afford. Remember that ideally, you should have some mechanical knowledge, or very deep pockets, to run any classic car.
Then, as said, use it regularly - I have always aimed for at least 30 or 40 miles at least once a month throughout the year, providing the weather is dry and, in the UK, there is no salt. I don't, by choice take the car out in wet weather - that said, over our 32 years of ownership, the old '78 has seen every kind of weather, save an earthquake!
So.... Keep it clean, inside, out and, underneath! Driving and cleaning are NOT mutually exclusive.
Service it every year as per the manuals, mineral oil, and a filter, grease, plugs etc. Check distributors every few years (HEI's don't need much except to be clean) Change coolant and brake fluid every few years, trans fluid and filter perhaps?
Don't expect it to behave like a modern car - that's the fun of them! If you want C6/C7 performance and handling, buy one of those.