If for casual wear pants (khakis, jeans etc...), try ZA Altera @ Mid Valley (1 level below Carrefour/GNC, beside the car park payment counter). The ladies there are pretty good tailors, price are reasonable too

If for business wear pants (slacks etc...), try Sparkmanshop @ 1 Utama Old Wing (Ground floor, walking distance from Jusco departmental store). I find them alter business wear to be better craftmanship

Say you've a pair of jeans that you want to alter the length only, it's about RM 10 - RM 15. They will keep the hem of the jeans if requested, so that even the length is altered, it looks very natural like it was never altered before. They cut the hem and reattach the hem to the shortened jeans.

If you want to reduce the hip size of the jeans, then it cost more, around RM 30.

They have a menu of the cost of alterations displayed at their shop, although I can't remember it but it's all reasonable.