Don’t be stupid, and waste your crystals right when you get them
Even at level 40, this is something I struggle with. Try to always save your crystals to at least 225, to get a Summoner Pack, or to 750, to get a Premium Pack. Although this doesn’t influence your luck at all, it is just more beneficial to you in the long run. Also NEVER spend crystals on refreshing the Temple of Wishes, the chances of you actually getting something good is extremely unlikely. Another good use of crystals would be to save up to 200-400 crystals and buy a 1 or 3 day EXP booster, then spending some time at somewhere like faimon and refreshing your energy to powerlevel some monsters. This can give you a nice boost if you are struggling in Cairos or ToA, and you should be doing this every now and then.