The chain on my winter bike had been highly neglected and left covered in the salt and snow most of the winter. Recently in attempt to save the chain I ran into some difficultly. I worked out most of the issues, but had about 4 or 5 links that would stick tight no matter what I tried. I checked on here but found nothing of use. What I came up with is this:
Knowing that I just needed a small gap between the outer plate and inner plate (visibly this is what was causing the link to not rotate freely) I thought heck, why not stick something in there as I use my chain tool. Having an empty box of nutty bars next to me I figured heck, cardboard is thin, just about enough to give sufficient clearance between the plates. So I pushed out the pin, separated the links, put cardboard inside the outer link, and reattached the chain. Then I soaked it in water/cleaner for a while and took it out. At first the link is extremely tight, cardboard expanded with water or something, but once you work out all the cardboard the link is loose as a goose. This worked on all 4 or 5 of the links I had been fighting with. Ghetto, but effective.
Note: This is not a problem if you:
1) have a chain tool with the 2nd slot used to loosen plates.
2) have arm strength greater than that of a 14 year old girl and can bend the chain at tight spot enough to separate the plates (not i)
If you have neither of the above and more time than money, this may just work for you also.