This may be common knowledge, but I see a lot of people spending money on straplocks, so maybe not.

If you're worried about your guitar or bass falling off your strap, here's an easy and cheap suggestion. Go to a hardware store and buy a couple of large rubber washers. The ones I use are about 1 1/4 inches (about 3.2 cm) wide with a hole slightly SMALLER than the strap button. They cost about $.65 each (USD). I put on the strap, then stretch the washers over the buttons.

The red washers that come on the tops of Grolsch beer bottles also work (plus you get a nice Dutch lager out of the deal).

I've also heard of people using the plastic clips that come with bread bags. I never tried that. I'm happy with the washers.

A chain and two large screws will do the trick too. Depending on the chain you can still use the original screws.
You can put some rubber rings inbetween the screws/chain and your guitar if you care about the finish of your guitar.

i go with rubber washers from bottles (grolsch etc). works like a charm.
cheap too!

A friend of mine swears by those little plastic tabs that keep the bread bag closed

Until I got smart and started using screws and such, I relied on duct tape.

DiMarzio ClipLock strap. Problem solved.

can someone explain that chain/screws thing?

It's not as uncomfortable as it looks.

I've had my guitar and bass strapped up like this for 5 years or so. Wouldn't have it any other way.
Just gotta tighten the screws every two months or so.