A simple technique that helps to create ideal impressions is to use Scotch tape on the outer aspect of your slotted disposable trays. First, I suggest using Teledyne Master Trays, because while they are generally appropriate in size, they can be altered by heating the plastic and bending into the desired position. Once cooled, it will not rebound.
The use of tape prevents the impression material from continuing to ooze out of the slots. The tape stops the material and diverts it laterally to the outer aspect of the tray. I always brush all the internal aspects of the tray with adhesive as well, to help secure the material.
In addition to using Scotch tape on the outer aspect of the tray, I also place it on the distal aspects of the tray. Placing tape in the hamular notch and the retromolar pad areas prevents the impression material from oozing out, and confines ther impression material, minimizing the chance for pulls or distortions on the distal most tooth.