I've had this issue as well...
I've tried several solutions:
Dip Deleter pen with ink bottle (not convenient at all)
And found the Sailor Profit brush pen (previously discussed on Sketchcrawl by Russ Stutler)
to be the best.
I ordered it off of his recommendation alone.
I keep it with me for impromptu sketching and it is a mainstay in my kit.
Immediately waterproof once it has dried and had enough time to "sink" into the paper. Roughly 5 minutes depending on the thickness of the paper.MangaTaro's Brush Pens (4 available)
He has other Sailor Pens available as well, as well as hard to find items from Japan. This pen comes with 6 refills and an extra brush nib.
With moderate use, this kit is really well priced and will keep you going for quite some time, and an extra nib if you find yours mangled or won't stop curling upwards.
I use it with my Moleskine Watercolor and have had really great results so far.
I purchased mine on Ebay, but if you're wary of Ebay, you can try Pen City