don't precook the fruit, do shake plenty of sugar over it , and a spice to go with. If it's not a "juicy" type of fruit then a slug of alcohol won't go amiss, or fruit juice if you abstain!
use more butter than you think reasonable ...at least equal to flour
some white sugar in the crumble mix
demerara on top
Fancy options for the crumble....add same spice to crumble as you did to the fruit
add a skimpy handful of porridge oats...do not go overboard or it will get gluey
BISCUIT CRUMBS....we once went to the Dean's Shortbread factory and they had bags of shortbread crumbs on sale ...I thought for cheesecake bases as a change from digestives, but the woman there said they were great in crumbles...and they really are...I often bash up Amaretti or Nice biscuits, or Hobnobs
My favourite crumble is Belle Helene Crumble , my own invention! ....pears in the base, tinned or soft ripe ones. (if you have ones like bullets then half poach them, this is the only exception to the "do not pre cook the fruit" rule)
When making the crumble add a couple of tablespoons of cocoa ...you can do this at the beginning or the end, but if you are making the crumble by hand the cocoa will get under your fingernails and ruin your manicure :o so I stir it through after rubbing the butter and flour together.
Nice generous splash of pear brandy or creme de cacao on the fruit and some chocolate chips stirred through the crumble, serve with chocolate custard and creme fraiche :o