ive got a 1999 6.5 turbo diesel what would be the best and cheapest way to get good fuel milleage?

 

You've probably heard this before but:

1. Your right foot. Control it.
2. Your routes, plan them.
3. Your trips to the store/etc. Combine them.
4. Warm up time, Minimize it
5. Tune up. Do it. (air filters, fuel filters, alignment, etc)
6. Synthetics. Switch to them when you come up to scheduled service. Can be a net loss if you spend more on fluids than you save on fuel.
7. Tire pressure. Maintain it to spec. Check it every weekend.
8. Drop any weight you don't need to carry. IE: crap in the bed, tools, etc.
9. Avoid lift kits, big tires, body pieces (sunvisors, wings, aux lights, light bars, etc).

After that, you're getting into serious cash, as in the multiple hundreds....

 

Gear it to spin 1800-2100 rpm at the speed you usually drive. They get the best bsfc there.