Legalise cannabis for over 18's, allow companies to cultivate crops and sell (including VAT) for less than the going rate on the illegal drug market. Price the dealers out of the market. Create kite markings, and regulations so people know their product is as safe as can be.
Reallocate the funds used on police resources to arrest, put on trial, and jail cannabis users, to thorough and transparent scientific research on the long and short term effects of cannabis use. Use this information to make a campaign, creating awareness of the results of the research, and discourage the smoking element, suggesting alternative methods for consumption e.g. tea or cookies (low fat mind you).
Create a system similar to Amsterdam, allowing a set limit of cannabis per customer from licensed cafes/bars.
Legalise to avoid tarnishing otherwise law abiding citizens with a criminal record, to enable those who get relief from pain to do so easily, and to give people a choice about what to put in their own bodies.
Why is this idea important?
It is important because prohibition has clearly not worked, the underground drug market is thriving. People are smoking weed with sawdust and shards of glass in, as the dealers do not care what they put in it, as long as they get their money.
People are still not discouraged from using cannabis, despite the obvious health risks of buying from an illegal source, so instead of pushing the money into the hands of obvious criminals, generate some money from legalising and taxing it.