The most common way that instant winners are selected works like this: before the giveaway begins, sponsors pick a random winning time for each prize. The first person to enter the instant sweepstakes after that winning time has passed will receive the prize.For this reason, instant win sweeps are one of the few types of sweepstakes where entry times matter.

The more difficult a sweepstakes is to enter, the fewer qualified entries they will receive. If you have to answer a difficult question correctly, or if you have to spam a hundred of your closest friends, you'll have better odds. Difficult sweepstakes are pure gold, but be sure to separate difficult from unrealistic (that is, you have to spam 100 of your closest friends and they all have to enter their email addresses on a website).
Space out your entries. Assuming that the number of times you can enter is constrained, either by the rules or by your budget, don't send in or drop in all your entries at one time. Wait a few days or weeks (depending on the contest) between entries so that when the entries get mixed together you've got a better chance of one of your entries landing on top.