Like buying any type of bike part you have to remember the rule of 2...

Cheap
Light
Strong

You never get all 3. You have to choose 2.
If you buy lightweight parts that are also strong, you are going to PAY for them.

Aside from drilling or taking off your brakes, the only way to save weight is to buy new stuff...

A cheap way to loose a pound and-a-half...

Lets say you're running budget Kenda street tires that weigh in at 2.75 lbs for the pair.
For $80 you could shave 1.5 lbs of weight in the most important place (the wheels)
by switching to KHE street tires.

Forks...Let's say you're running a set of 3.6 lb S&M Ditchforks from 1998.
For $100 you could swap them for Eastern Ultra Slim forks and shave 1.6 lbs.
Your forks may not be as heavy as my old ones were, so there may be somewhere else you can shave more weight for less money.

Wheels: There is a local bike shop that sells primo mix cassette rear/primo front laced to primo balance rims /w double butted spokes. Their price is very cheap.
If you PM me I'll give you the contact info and price. I haven't weighed this wheelset, but I estimate it comes in around 4.5 lbs. Depending on your current setup, you could shave significant weight by changing the wheels.

Obviously there's the frame...but that gets expensive.
I had an S&M next-gen dirtbike that weighed 7.8 lbs.
I was lucky to find a used Metal dagger for $50.
That frame is 6 lbs which is heavy by current standards.

Compared to my old setup: I was able to cut out just 2.8 lbs on my new build with a used frame and a new federal fork.

With my new build, I replaced Frame, fork, wheels, and chainwheel.
I Shaved 5 lbs total off my old setup, for about $450 in new parts.