If the van was working good in the first place we wouldn't have tips, however...
- never drive below 1/4 fuel gauge because gas cools electric fuel pump, an $800 part
- add can of solvent to fuel tank to fix fuel gauge erratic behaviour
- watch front wheel alignment - replace idler arms/ball joints with higher quality "moog" parts
- caulk around rear doors to prevent water leaking and building up under carpets
- replace hoses in vacuum system if fan/heater control isn't working
- grease front end regularly
- change signal flasher unit if turn signals are erratic
- don't touch the electric mirrors unless necessary - they don't move very well, and it's a pain to replace a cracked $70 GM mirror face
- if you have starting problems in cold, damp weather - the block heater may help, change cap, roter, spark plug wires, coil and battery
- use 2nd gear to avoid spinning on startup in snow.