Don't take everything you see lying on the ground like a fucking gypsy. Decide the loadout you're going to use and stick with it - then be very selective about what you pick up according to two simple rules of thumb:
1) Is it valuable?
2) Will carrying it/these fuck my weight to a point that comprimises me if I meet a Chimera?
That's the script I use in my head when playing ironman, and it helps.
Regarding Rule 1, as this user said, don't bother picking up peasant weapons like M4s and low-tier AKs. Unless you're modding the 'traders don't buy damaged weapons' feature out, you have to repair weapons before selling - so only haul back MGs, modern bolt-action sniper rifles, and 7.62x51 calibre rifles artifacts which fetch ~10,000RU after a repair. You'll make between 9 to 8k profit from each.
As per Rule 2, agreeing with this user , using bags to create caches is good practice. Sometimes you come across a pile of loot but it weighs too much to drag back in one go. Stash it away and make a couple of runs rather than being the greedy cunt who gets pounced by a pack of snorks and can't run away.
Weight management comes down to getting the basics of your equipment right. Regardless of the mod you use, that generally means a shotgun for mutants and a (read singular) rifle for humans. If you like AO, a Vepr/Saiga and the best AK variant you can afford in 7.62x39mm with a grenade launcher is hard to beat.
>VOGs for quickly fucking high-tier mutants
>Ammo is plentiful from corpses and reasonably cheap
>Effective against unarmoured, and effective enough against armoured with AP rounds
>It's not the best at everything, but it doesn't leave you with any extreme weaknesses
With upgrades on weapons and a decent suit, you should only be around the 30kg mark with 90 AP rounds, 30 buckshot and 5 VOGs at any given time. That leaves you plenty of space for picking up high-tier loot from corpses.