I have a 7mo old who was having the same problem until about three weeks ago. We discovered that if we could get in at least 24 ounces of formula before 10pm that she would sleep through the night. She has her last bottle at 7 or so and is usually asleep by 8. And then I started waking her at 10, well really just picking her up and giving her another 2-3 ounces. She usually just goes right back to sleep. That's worked for us. I also heard that if you slowly decrease the amount you offer at night or even offer water then they will lose interest and stop waking up.
Here is an article I just read from whattoexpect.com: If your little night owl is hooting for attention from dusk till dawn, chances are you're a tiny bit tired. Or more accurately, exhausted! At seven months, there's no real nutritional need for night feedings. All babies will wake throughout the night, and most will eventually learn to fall back asleep on their own. Every baby is different, but if your wee one is clamoring for comfort at 1 a.m., these strategies can help minimize those wake-up calls.
First, fill 'er up. Make sure she finishes her last feeding of the evening before dropping off to sleep, and that she's getting enough to eat throughout the day too. You might even try gently waking her just before you go to bed for an extra feeding. (Yes, you should let sleeping babies — and dogs — lie but drastic times call for drastic measures!) The idea is to maximize the number of calories she takes in during your waking hours, so she doesn't need to interrupt your sleep — and hers — for more food. If she continues waking, chances are at this stage, she's searching for comfort. So instead of offering the breast or bottle right away, attend to her in other loving ways: pat her, cuddle her, or — better yet — send Daddy in to soothe her to disassociate nighttime with mealtime altogether. Another disincentive to those night feedings: Make those undesired night feedings, well, undesirable, by shortening nursing sessions or cutting back (or diluting) the formula in her bottle.
Second, make it boring. Don't give your baby the idea that nighttime is playtime. Dim the lights (or turn them completely off where you can), and keep bubbly conversations and sing-songs to a minimum.(Soft lullabies are the exception!) And unless her diaper is especially dirty or sodden (or if she's prone to rashes), leave it on her until morning — eve a diaper change is just too much activity!
Third, determine how much crying you can take. You have a couple of choices: You could go cold turkey and let her wail through a waking. She might just give up and go back to sleep on her own after a few minutes. If she doesn't (or you feel uncomfortable with this method — many parents do), try a more gradual approach. Respond, but don't feed her. Try soothing her without picking her up from the crib. Leave the room and let her cry for a few minutes, gradually lengthening your absences. Remind yourself that you're not a meanie! You're just helping her learn how to soothe herself to sleep — a skill you'll both appreciate for many years to come. But if you don't feel the need to push the independent sleep agenda at this early age, that's okay, too. Many parents find co-sleeping to be a viable and peaceful alternative well into toddlerhood.
Whichever route you choose is a personal parenting decision. And rest assured that regardless, all kids eventually will learn to sleep on their own, and voluntarily (most often between the ages of one and three). So any way you slice it, this, too, shall pass.
Good luck