When X and I first started having playtime, oh so many moons ago, one mistake I made was assuming he needed to be "entertained" all the time. I'd tie him up, like he wanted, but then I'd keep myself crazy busy continuing to stimulate him. I'd run out of ideas, I'd get frustrated, and I'd end the scenario before either of us was really satisfied.
But guess what? If your lover likes being tied up, this is a stimulation in and of itself! I've played with the timing of it and I've learned that I can tie X up and leave him completely alone - not even be in the room - for 15-20 minutes. What do I do during that time? Anything I want. Finish watching a show. Read some erotica or watch some porn to get myself more in the mood. Masturbate so I'm finished and can better focus on him. Call my mom. Anything you want.
If you want to try tying your lover up and leaving him alone, there's a couple things to keep in mind.
1. If you're using new restraints or a new restraint system, it's better to stay in the room just to make sure your lover isn't uncomfortable or in pain. You want to tease him, not torture him.
2. X likes the idea of being tied up and left alone, but in practice, he likes to be able to call for me if he needs me. To that end, I never gag him when I plan to leave him alone. I tie him up and blindfold him, but I leave his mouth free and I stay in the next room just to make sure I can hear him.
3. You can do a lot with "check ins." Say you're trying this for the first time. Tie your lover up and leave him alone for 3-5 minutes. Come back in and talk to him and touch him. Make sure he's still riled up (he will be). Then leave again for 5-7 minutes. Repeat. You can keep someone going for easily a half hour this way and it's not boring at all. It keeps his interest piqued with minimal effort on your part.
This can also work really well if he's interested and you're not. So your lover wants to play. Fine. Tie him up and do something to get yourself in the mood. Read or watch something sexy. Touch yourself. Touch your lover. But he's a captive audience and will be good and ready for you when you are.