Hi Grasshopper - I've been riding for about 2 years but it wasn't till the start of this year I was able to stand out of the saddle. I had the same problem - I could manage a couple of strokes then my legs would just collapse.

Ealier this year I started spinning and was also totally unable to stand, but after a few weeks I finally got the hang of it. I was so pleased! I think it was helped that my legs had got a bit stronger, but also down to improved confidence, balance and bike handling.

The more you ride and get a feel for your bike the better, so your confidence will improve and you'll soon be standing out of the saddle.