I had the same issue with my son who is now 5 months old. I found many solutions while going to physical therapy starting two months ago (he had tightness in one side of his neck) but I'm sure some of tips can help you too.
First thing, when your baby is on their tummy, the physical therapist had me hold the arms in so that they could be on their forearms to gain strength in their shoulders and neck. Kind of like doing the yoga pose for cobra. (have their upper body lay over a pillow or boppy until they have really good strength... it makes it easier for them to be angled up when holding their head up)
Next, they used one of those exercise balls in front of a mirror. The ball allowed you to hold them at different angles also designed to help them get used to tummy time while they gain strength plus bouncing helps keep them happy.
Finally and what helped my son enjoy tummy time more is to have toys that make sound, lights and have motion like the Vtech Learn and Discover Driver toy that you can find at Target AND changing positions. While playing on his tummy we would also roll him to his back for a little while, then side and back to tummy (never really having to pick them up... the roll was fun for him too). I noticed he might start to fuss on the tummy but would go back to enjoying the toy when changing position. I think the repositioning helps keep their interest.
These tips have helped considerably and my baby doesn't need physical therapy anymore but he doesn't enjoy his tummy soo much that we leave him on it for more than 15 minutes before he gets bored and crys... I'm hoping the frustration also encourages more rolling on his part.... he's soooo close.