Depends on what type of bike - hardtail or fully... this is OK advice for a hardtail but not so good for a fully. Watch competitive riders... you will see most stay in the saddle (even those riding hardtails) because the bike works better that way. Key is to develop an even and efficient cadance and pedal stoke - so as not to spin out on gravel, dirt or sand. That generally means going into a lower gear. Remember its totally different than road riding 'cause hills are steeper and terrain is loose. Low gear, efficient, smooth, consistant pedal stroke, butt waaaay forward on the saddle, chest bent over the handlebars. It's uncomfortable and takes practice but after a while climbing straight up becomes eas(ier).

Note to add: Stop "attacking" the climb. I bet you spin out alot and lose control. Next time try adjusting the gears to something lower and just "rest" up the climb, working on stroke and position. Once you get that down, then increase gearing and force. You will be amazed!