Besides the advices allready given you could look at garage sales and Ebay and sites like that for used collections.
You will get a lot of different bricks that you didn't know you needed instead of going to Bricklink and buy the 200 yellow bricks you know you miss to build a house. Yes you will have some bricks with scratches but you can always use them later on as "filling" when your collection of sets that you have parted out is big enough to build what you want.
When buying at Bricklink then browse the rest of the categories and buy some stuff that you might need at some point... that worked for me
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Jan