spamming slide before a jump never seems to build up more momentum, also any good levels to practice like the sliding trick or a GDJ

You don't spam slide before the jump. If you're talking about the SJS, you do not spam slide, you slide and while sliding jump and uncrouch, then crouch again to do a crouch jump. Level 8 is a good level to practice SJS, as well as level 3 and level 0, as for GDJ, level 1, level 2, level 3, level 5, level 9, and level 10.

Ahhh thx very mucuh, I thought you had to spam slide