I've just started buying rockmelons and they were only sweet a couple of times. The other times, they've been very low in sweetness and it felt like a waste of money. Are there any tips when buying melons:

1. Which melon to buy - rockmelon, honeydew, etc.?
2. Should I knock on them for a particular sound?
3. Should they be a particular weight?
4. Should I only buy them in a particular season?

I did look at this link, but we don't get these varieties in Australia, at least where I live.