I have read a lot of people say point your toes down or push forward on the bars... Really, what they are describing is an endo.
After lifting the front wheel in the air, what you want to do is an endo... Only you aren't on the front wheel, it is in the air, which means you REALLY have to know how to do endos.
It has always been tough for me to describe bunnyhops because I had endos completely dialed before I started trying bunnyhops, so I was hitting about a foot or so in my first week of trying bunnyhops.
I really think there are a lot of little things you can do to learn the motion and to do bunnyhops.
1. Learn endos - use a curb to stop your front wheel, jam your foot in the front wheel (not as good), or just learn to push forward on the bars and point your toes down and lift the rear wheel. You should be able to EASILY lift the rear wheel enough that you flip forward onto your face - if you can't, or struggle, then you don't have the proper motion for endos, and you won't for bunnyhops either.
2. Hit a small jump - like 2 or 3 inches. The angle at the end of a driveway works, but nothing BIGGER than 3 inches - 1 inch is fine. When the front wheel hits lift your front wheel, then when your back wheel hits, lift it. Practice that for 15 or 20 minutes a day. This improves your timing and stops you from the nasty habit of lifting both wheels at once - BAD habit.
3. Work on bunnyhopping over things... Start small and safe, but really, you should be clearing curbs in no time, then when you are at the 7 inch mark, try to clear some distance. A sidewalk with curbs on both sides is a good goal. You go fast and work on your technique, and you may eat crap occassionally. Or maybe it was just me.
4. To tuck - you don't. Okay, yes you do, but it really is something that happens once you are comfortable with the motion. When you can lift the bike about 12-18 inches regularly, you will find that the motion is second nature. Then you will bend your knees. Just like you do on a really high endo. You bend your knees and tuck the seat in front of you. Extend your arms out in front of you... It all has to happen VERY quickly in one smooth motion, so it is not something you just 'do'... it is a skill you learn. Probably my best was bunnyhopping over a garbage can from flat ground to flat ground - so about 36 inches tall and a couple feet across. I was a lot younger then though.
5. Most of all - give it time. It isn't hard to get to a reasonable height very quickly, but odds are good that you don't have all the muscles or techniques perfect for high bunnyhops. It may take a month to get to a foot - then 5 months to get to 18 inches... Then a year or more to get another 6 inches. But, you will always improve as long as you are in decent shape and work on it everytime you ride. Eventually the bunnyhop is less something that you 'practice' and it just becomes a tool that you use for a lot of other tricks.
For me, as a flatland rider, I could do bunnyhop 360's and 180 bunnhops to rollbacks. I never did a whole lot else with them. I learned abubacas on stuff - mostly curbs.
Have fun