- Consider how you're posing the model
- Closest to the manufacturer's pose should be safest.
- Have as much of the base under the model as possible
- If the model is leaning forward, move it near the back of the base so it's leaning over the base.
- If it's falling backward, move it near the front of the base so it's falling back over the base.
- If the model's standing mostly straight up, put it near the center of the base.
- Lower center of gravity
- Move the heaviest part of the model closer to the base.
- This is usually around the torso, but as you can see in the model right, all the weight's along the shoulders.
- Weigh the base down, as covered below. The heavier the base, the less likely a model is to tip it.
The first way is all natural, and involves making the terrain on the model's base out of real stuff like rocks and metal. This has the advantage of being able to add a lot of weight to a base because you don't have to hide the objects weighing it down.
Some rocks and metals are heavier than others, so keep that in mind when choosing your materials. If you're raising the model up and putting weights beneath it, beware in-game negatives this could create. GW line-of-sight is (was?) to body mass, so raising a model up makes it easier to shoot, while PP los is to a vertical volume of the base.
The other way is to hide something heavy under the base. This has the draw-back that you don't have a lot of room for weights since bases are so thin, so you're not adding too much more weight, but anything can help. It does mean you can do whatever you want as terrain on the base.
Your options are basically limited to using coins or pins. Coins are easiest, but sometimes thicker than the room under the base and some bases don't have room. (As pictured.) You can also cut a bunch of short pins and glue them underneath. Make sure the pins aren't too crooked.
You can use other stuff to fill the bottom of a base - glue, green stuff, resin, filler putty, etc. Just make sure the bottom of the base remains flat. You don't want it curved or bumpy, else it might get more tipsy.