How can i make sre the saddle doesnt slip!?

i have a youngster horse, who's back has not finished growing. we've had many saddlers out to fit him with saddles and the one he is in now is correct. but still, i do the girth up loads as tight as it can be with out being worked in and hes saddle still slips when i try to mount and now ii cannot mount from the ground with out someone else being there. the mounting Block is no help either as he is afraid of it and it take ages to finally get on, if anyone has any ideas i would be very grateful!