Going from good fit to "just right" fit

Okay, I'm not a fashion expert, I'm new to this board, and I don't have a closet full of designer clothes. What I do have though is a fair selection of decent quality off-the-rack suits that come close to fitting, but don't quite get there--and that is driving me nuts. To me, and everyone else here I'd bet, fit is everything, and improper fit can make a great suit look like a potato sack, and great fit can make a cheap suit look, well, like something better than a potato sack. It seems that either a jacket fits me perfectly through the chest, but the sleeves are an inch too long. Or a jacket seems to fit perfect through the back and looks fantastic unbuttoned, only to have a bit of a gap in the lapel when buttoned. Or the length is just a bit too long and makes my legs look shorter than they are. Incidently, I'm about 5'10", usually find that suits labeled 41 or 42R fit me as well as I can get, and I have a about a 33-inch waist and an inseam of about 31.5. I really don't think with these basically average proportions I need to go the full-blown bespoke route to get a good fit. Does anybody truly get a perfect off-the-rack fit?

What alterations can one reasonably entrust to the much-derided but ubiqituous "alterations lady at the local dry cleaners"? Are sleeve lengths on a jacket safe? I guess I've come to realization that I actually do have a fairly solid, basic wardrobe in the way of suits if I could just get them to fit that little extra bit better. Going to a full-blown tailor who makes suits from scratch seems like overkill for my RTW wardrobe, but hey, I'm to the point that I just need some help and advice on fit. The lady at the dry cleaners might be able to help with the simple stuff, though, like sleeves. Right? What I'd really like is somebody with some knowledge to look at me with my suits on and tell me what's working and what isn't, and suggest how I might improve things. I guess that's a tall order and likely expensive, but maybe it's time to bite the bullet.