As other's have mentioned you can get better at riding out of the saddle by doing just that. It sounds trite, but it is what it is.
If your current "max" is 60 seconds then simply do 2 sets of riding out of the saddle for 25 seconds at a time and do that 3 times (2 Sets of 3X25sec). Spread these out over your entire ride. Now you have a total of 2.5 minutes of out of the saddle riding "practice". You could do that same kind of practice with out of the saddle riding for sprinting as well. Maybe not as long of periods of sprinting but getting out and doing it more in a kind of progressive manner will help a great deal.
Good luck.