In reply to Kirill:
I think the main way to improve is consistency, so staying injury free is key - only takes one injury to knock you out for months and then you make several steps back.
But I agree with you regarding frequency - once a week simply isn't enough. You can try mixing the sessions, it should help eg alternate between training endurance, power-endurance, power and maybe general fitness/conditioning.
I remember reading a book on climbing and if my memory serves me right, the max power training frequency that was recommended was twice a week - so if you are a relative beginner (like me), don't think even one power session a week is unreasonable for a while until you can build up.
Also it's not really mentioned but I think sometimes the 'easier' bouldering problems can be back for elbow and back injuries because naturally beginners can't use small holds so always go for 'juggy' routes that allow for a LOT of pulling if done with bad technique. I find that when trying slightly harder problems, the main issue is fingers but they seem kinder to my elbows and back... (probably also because I usually just do 3 moves before falling lol)