1. Keep a Workout Journal. A workout journal is the only way to make sure that your bench press is improving. Each week you should be writing down the amount of weight you're pushing and repetitions you've completed for each set. Note how many repetitions you were able to do on your own as well as any forced repetitions that your spotter helped you with. You should also be referring to your previous workout, always aiming to increase the amount of weight or repetitions you complete.
2. Eat Right. You can't increase your bench press without proper nutrition. You need to make sure you're getting enough calories. To improve your bench press, you need to build muscle, and to build muscle you need to create a calorie surplus (eating more than you burn). Get your calories from clean foods. Don't eat too many foods high in sugar with the exception of your post workout meal/shake. Most of your carbohydrate intake should come from complex carbs - 100% whole wheat breads/pasta, brown rice, oats and vegetables. Eat regularly, every 2-3 hours. Eat a light meal about 1 hour before you workout. Make sure that your meal right after a workout is high in protein and sugar, this will allow your body to quickly begin repairing any muscle damage that occurred during your workout. Don't forget about fat. While some people go on a very low fat diet in hopes of reducing their body fat percentage, fats play an important role in health, don't leave them out of your diet. Your fat intake should come mostly from unsaturated sources - fish, nuts, oils and vegetables.
3. Don't Bench too Often. While it may be tempting to bench as often as possible for maximum gains, doing so will only set you back. The bench press is a very rigorous exercise. Each time you bench, you're actually creating tiny tears in your muscle fibers. When you rest, your body repairs this damage which is what makes you stronger. If you're benching every day, your body won't have the time to repair the damage and in the long run, you'll see your performance suffer. You should give your chest at least 4 days of rest before working it again.
4. Other Exercises. When you're looking to improve a certain exercise, you naturally want to do it over and over again, practice makes perfect, right? While benching will obviously improve your bench press, you shouldn't neglect other exercises. The bench press is a compound workout - it involves more than one muscle group. If you want a strong bench press, you need to look beyond your chest. Your shoulders and triceps are heavily involved in benching. Having a good shoulder and tricep routine will help you achieve a stronger bench press. You should also be doing more than 1 chest exercise. Incorporate both barbell and dumbbell bench presses, flys, push ups and even machine presses. Using different exercises will help your chest get stronger through the entire range of motion.
5. Use a Spotter. Using a spotter will allow you to do more repetitions. When you're done doing as many repetitions as you can, a spotter can help you with forced reps. These forced reps are achieved when the spotter gives you just enough help to get you through extra reps.