As one of our local experts in getting cracks (wonder how that could be ) here's my advice:
- be careful when taking your armor off. Seconding @andylah here, cause I've gotten a few cracks when the snaps got stuck or I was careless
- be careful when moving. You've likely got a more limited range of motion when in armor than outside of it. Now, don't let the fear stop you from sitting down! But move slowly and avoid "rolling" over your armor (for example, sitting on the front part of your buttplate and then shifting your weight backwards). Sit straight down, adjust carefully.
- if your armor pieces get caught/stuck, don't remove them with force. It won't ever turn out well.
- there are a variety of repairs. Fabric with superglue is easy and fast (ideal for at a troop!) but it's the most likely to crack again, since it's really thin. ABS paste also doesn't tend to hold up well, so only use it for filling up gaps or cracks. Instead I recommend pieces of ABS plastic glued into your armor. If you've shaped them properly (curved to fit the armor piece, a bit larger than the crack) they almost never open back up again, and even if they do, it's highly unlikely to become bigger.
- don't bend your armor or put stress onto it, if you can avoid it. Bend your ABS often enough and it'll crack.
- remove unnecessary return edges in high stress areas: I recommend no return edges on your buttplate if you want to sit down in armor. Not only is it more comfortable, but without a return edge the armor won't crack as easily.
Personally, with my next ANH armour I'll be removing the buttplate return edge and I'll put some ABS into the codpiece, buttplate and chestplate arm holes, since those were my common crack areas. Just don't let the fear of cracks stop you from having fun! Checking your armor after every troop and repairing cracks as soon as they appear will help you keep your armor looking good.
Hope this helps!