1. Don't use pre-filled cartomizers anymore. They're usually bad-tasting. Buy pre-mixed juice and try to pick flavors you think you'll like.
2. I found the flavors I liked within a week. They happen to be RY4 and Apple from FreedomSmokeUSA.com. I mixed them together and that's what I've been vaping for the last four months. That's all I vape all the time, except I also started adding Caramel and Nut Mix (both from FSUSA) in mid-October. It's a great taste, but I'm not recommending you to try these flavors. Just letting you know what worked for me. RY4 (or any of the RY juices) are different from each supplier, and I happen to think RY4 from FSUSA is the best RY4 (and the best juice when mixed with Apple, Caramel, and Nut Mix pre-mixed juices).
3. Cleaning cartomizers - It works fine when they're new, but after they've been refilled several times, cleaning them doesn't help much (it actually degrades the filler inside). After about 15 refills, it's time to throw them away (when I used to use cartomizers, I quit trying to clean them, because it was a waste of time). If and when you clean them, you should use a 10mL syringe and rubber cone (available at Walmart, a children's medicine kit for about $1.76). This will allow you to push air through the fillers to remove all the excess water, after you've plunged out the old juice.
4. Drying cartomizers - Place them on a flat pan in the oven at 220 degrees for a few minutes. This works great and is the best and quickest way to dry them completely.
5. When you get tired of using cartomizers, consider using just an atomizer and drip tip (as a mouthpiece, actually). The flavor is better, without having to taste the filler, it's quick and easy to drip in 3 drops (which is about one cigarette worth or about 9 drags), and then you won't have to buy blank (or empty) cartomizers and mess around with refilling them, waiting overnight for the juice to mix in with the filler to get better flavor, and wondering if you should clean them. I used to think using atomizers would be a hassle, but after three months or so, I discovered that I should have just started with atomizers. Once you get the hang of using atomizers, it's so much better.
Good luck! It's worth it to hang in there and keep looking for the juice you like.