Although everyone will tell you to take things on an empty stomach, I think both an empty and full stomach have their advantages.
On a full stomach it takes longer for your drug of choice to be absorbed. This is good or bad depending on what you're looking for - the effects tend to be a bit weaker, however they also tend to last a little longer. I've used this little method to extend psychedelic trips - taking my dose right after a meal.
It's also useful to eat beforehand if you have trouble eating while high, as it saves you from going without food until after you come down.
That said though, in general if you want it to hit you hard, keep your stomach empty