The best way to stock up on free travel-sized toiletries is probably to take a cue from Hoarders. Hoard, hoard, hoard.
So, like, short of having an addiction – I mean, I assume you do have a travel addiction of some sort or you wouldn’t be on this site – the best thing you can do is gather every single free toiletry you can get your hands on. Most often, free samples are just given away at makeup counters or at Sephora stores: these work really well in keeping under the 3-ounceish liquid limit rule.
If you’re ever at a hotel, make sure you pick up those extra shampoo and conditioner bottles for those carry-on flights. Keep a cabinet of them at home so you always have a stash, and when it’s time to go, toss a new bottle in the bag. Finally an ingenious use for finally using those samples!