There are some fabulous suggestions here. Wish I'd have saved one or two to try them.
I was lucky enough to attend one of Michelle Zindorf's classes. I too, had bad luck with these. She did suggest to store them point down. She also said never to leave the cap off even for a minute after you are done. I've never had one write after 6 months. She did say they will clog. I gave up. I purchased Copic Opaque white and a glass dip pen. I diluted the Opaque white with just a few droplets of water and works like a charm.
Good luck getting them to work consistently.