This is a complicated question and the answer is multi-faceted. First, if you increase your 1rm the number of repetitions you can perform with 225 will increase. For example, if athlete A has a 1rm of 275, 225 pounds is approximately 81% of his max. Athlete B has a 1rm of 350, 225 is about 64% of his max. Athlete B will obviously be able to perform for reps because 225 represents a lower percentage of his max.
The second part of the answer is to improve muscular endurance. I have found when training my athletes for the NFL Combine or Pro Day that cluster sets are one of the best methods to train for this by manipulating the variables involved. Finally, proper set-up is a key component often overlooked when training for the 225 test. By manipulating the mechanics of the bench press you can decrease the range of motion and increase the number of repetitions you perform. Max grip, big back arch, and feet behind the knees (to keep your ass on the bench) are just a few of the points to look at during the set-up.
This post is already long enough without going into details of actual training templates, but I will leave you with this. At this time of year with The Combine and all Pro-Day's finished, why are you concerned with a 225 rep test? Unless you improve your overall strength, power, athleticism, and football skills, the scouts will not afford you the luxury of performing a 225 rep test for them.