That's not the 'key' to collecting coins. You have top follow the market to know what coins will increase in the future. For example, the $10/$25 gold proof kangaroo has done really well in the past and the 2010 edition did really well too. I think it's worth 2x it retail value already.
If a coin sells out in a few hours, as some have, then it's usually a good indication to get the next years edition as soon as it comes out and try and buy as many as you can as those type of coins are extremely liquid and will sell easily if you need the cash.
But my recent purchases have been mainly because i like the look of them. My favourites are the uncut notes - so nice to look at