Simplest method of strain relief I've seen is a knot in the wire that stops it being tugged through a hole in the casing, from that there are several methods, from moulding pegs or special openings in the case, using grommets, low pressure moulded features on the wire, screw down clamps...
Glue, generaly hot melt glue can be used, or silcone.
As for connectors, there are so many out there, by so many companies... Depending on the price of the product being designed will have a big inpact on where I'd look for connectors, and predicted volumes. For low cost high volume I would look at more diverse suppliers (China etc), for high cost/reliability known manufacturers (Molex, Amp, Samtec AB Connectors etc). Looking for a new flexi to PCBB connector set I spent two weeks just looking at what is out there these days!
Good luck, as headphones are a priime example of the weakest link, the smaller ones get so much abuse that they generaly break at some point, so they are for such a simple thing a challenge to design.
Have Fun
Marc