It took me a while to get a grip on the liquid in the mouth problem when I first started (the worst!). It isn't always the filler that makes the liquid in your mouth, it is the liquid that is on the side of the cartridge.
I found out that when I topped off cartridges, I would get a bit of liquid running down the cartridge when went into the air channels of the mouthpiece and get liquid in my mouth.
The solution was to get into the habit of gently blowing down in the mouthpiece each time I got ready to vape to clear it out a bit.
Then I ended the problem pretty much by transferring my liquids into small 2 dram (8ml) glass bottles with a squeeze dropper top and a glass tip. I didn't have the over filling running over the side and down the cartridge and I can suck back in any excess liquid I'd dropped in there before it ran over the sides and ran into the mouthpiece.
So, the problem was the the plastic droppers that most liquid comes in since it was too easy to add just one too many drops (which isn't correctable) which ran down the cartridge side.
You can find them on ebay. Here is a picture:
You also can find them at some aromatherapy or herbalist shops locally. Check out the web or your local Yellow Page listings. I pay about $1.80 here locally.
A list of cartridge modifications are here:
http://www.e-cigarette-forum.com/forum/ask-veterans/93586-best-cart-mods-510-atty.html
I think I tried them all (gah!) and got pretty tired of fiddly stuff and just use AquaClear now. If you cut it slightly larger than the cartridge (only the width slightly larger and compressed when you put it in) and cut the depth and length the same as your cart size it holds liquid well and wicks it properly. If you cut it too wide and over compress it, it won't wick well, but you can always trim it down. Inexpensive and reusable once it is cut and available in the aquarium section of PetCo and PetSmart.
The combination of the dropper bottle and the AquaClear has worked well for me in all the atomizer models I own (801/penstyles, 901, 510, 401).