The trick to getting higher with less is to use 'til you feel it. Holding it in longer could be a possible way of making you higher by either pot or oxygen deprivation. The plastic bag may not hold the smoke well enough and I hear most people say by that time, the smoke is stale and can't give you the effect you're looking for. Food can kill your high, but candy like a now and later last forever and make your mouth salivate enough to quench your thirst. It's all comes down to personal choice.