There are other ways to isolate these compounds (phase transfer catalysts), but an even better option is column chromatography.
But first, a word about decarboxylation. Whether to decarb can be debated forever, but if you do, it should be heated under nitrogen at 105C for 1 hour.
I like to use a non-polar solvent to extract and a polar solvent to wash. Besides butane, try propane, hexane, and chloroform (especially efficient). Absolute ethanol (200 proof) is my polar solvent of choice.
When you intend to isolate the individual cannabinoids you should stabilize the crude product. With the product dissolved in 100% EtOH before winterization, filter the solution through charcoal, extract with 2% sodium sulfate, then filter through charcoal again. This prevents oxidation of your crude product.
The odoriferous terpenes can be removed by steam or vacuum distillation. Cautious distillation in vacuo yields a fraction of crude red oil (bp 100-220° C/3 mm). This can be purified by redistillation or column chromatography. Use ethanol to remove the residue from the flask while it is still hot. Filter the solution through charcoal, and strip the solvent. Distill the residue to yield pure red oil (bp 175-195° C /2 mm). Distillation must be stopped if smoke appears, indicating decomposition.
Because THC is heat-sensitive, it is preferable to isolate the cannabinoids by column chromatography. The simplest method of column chromatography is performed with ethanol and ether extracts of hemp on alumina, yielding two major fractions: (1) chlorophyll, CBD, and CBN, and (2) THC. A second, more difficult method is performed on Florisil (use 10 times the weight of the oil) with the solvent system hexane:2% methanol. This yields a doubly-concentrated, viscous oil which can be repeatedly chromatographed on alumina to separate the THC and CBD. The CBD fraction of column chromatography can be distilled (bp 187-190° C/2 mm; pale yellow resin) to purify it.
