That's like asking "how do I get that fish to take my bait so I can catch him". The answer is you can't make them buy you have to figure out if your product (the bait) is appealing enough to skip the trial.

Do you have any customers who have bought without the trial? If so, ASK THEM why they did it. That will give you much better answers than the people here who are giving good, although generic answers. You need specifics about your product.

Perhaps you are asking the wrong question. The bigger question might be "how do I double my buy rate after they've done the trial."