You guys do all sorts of crazy rust chipping effects on your mechs. I wanna have good looking mechs too! How do I do that? Thx!

Easy Mode:
You get a sponge and you tear a bit off to get a nice rough edge then you dab a bit a bit of paint. usually its a dark and rusty or primer type color or a silvery metallic. Makeup sponges work well as do soft foam inserts from various gadgets.

Hard Mode:
Layer different colors of paint and apply Hairspray in between the different layers (make sure the paint is dry before you spray) . Then you can create chips easily using a wet brush to knock off the top layers.

There are a bunch of guides on youtube if you need more info or a demo.

TLDR:
Sponges and Hairspray.

I would lean more towards sponges than hairspray. Hairspray works because it is easily damaged and dissolved, which allows the paint to easily come off in chips. Good idea for a display piece that will rarely be touched, not so good idea for models which will see a lot of handling. A sealer may not be enough to prevent further damage, since the hairspray is already under the sealer.

The hairspray method looks very good, but it can damage easily if you are too rough with the models. Using a torn bit of sponge foam and lightly stippling edges works wonderfully, especially on areas that see a lot of wear. You can also "dot in" some chipping with a brush, but it takes a lot of time to get it right.

Softbatch said:

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Some times throwing them at wild pets or small children works but could damage the models as well. The sponge method combined with brush marks works great and is very durable for table play when sealed properly. I seal all my models with krylon painters touch gloss-2 coats, then testors flat-2-3 coats as needed. I have models older than i want to admit in perfect shape if stored properly. Use whatever method you think works best for your style of painting and time available. Good luck with your painting!!!