A year or so ago, I was rubbish at riding standing up. Either for short, sharp bursts or tricky climb sections etc.
I forced myself on almost every ride to stand up more & more whether it was strictly necessary or not. Me and a mate also challenge each other and ride long sections stood up, particularly uphill.
In only a few weeks I noticed a real difference in my ability to ride standing up when necessary.
In terms of climbing sitting down/standing up it normally depends on what you are riding up. I will often get out of the saddle for short, sharp blasts up slopes etc. but for most of the off-road climbing I do I try & remain seated. I find I get much better rear traction doing that. If I do need to stand up to power over an obstacle or something, then I stand, but am constantly trying to gauge how much grip my tyres have and alter my body position accordingly. Most of the time it works, sometimes it doesn't!