USE A NEW COOKER OR SPOON
If the cooker has been used more than once, then you are testing everything you've ever made in that spoon or cooker. Use a new spoon or new cooker to ensure you are testing a single substance.
DILUTE WITH WATER
If it's not diluted well, false positives are common. If you are testing Methamphetamine you should dilute with a FULL CUP of water (6oz-8oz, really) Otherwise false positives are common.