1) Playing games. Playing games is a key point to success in making
Neopoints. I try not to submit points in a game until I get a fair score (eg. like on Meerca Chase 2, around 1000 points would be OK). Also, I never play games with a very bad points to Neopoints ratio unless it is for an avatar. To me, it is a waste of precious time.
2) Collect Avatars. As weird as it sounds, sometimes avatars are a huge
investment. If it takes an item, you can lend it out for Neopoints or you can
just sell it. (Read more about selling - #4)
3) Buy, Buy, Buy! The more items you buy, the richer you will be. Does that sound funny? It's the truth! Buy items that people need like avatar items or battle items in wars, etc. and you can sell them for a higher price. Do some research, though, on what items are listed as HTS (hard to sell) and ETS (easy to sell). Also, consider a gallery. That way, when the items are worth a lot, you can transfer them to your shop and make tons of Neopoints with barely any work!
4) Snipe. This is what I do a lot. To snipe, you need to start out with a
handful of Neopoints. What you do is buy items fairly cheap. I sniped a baby paint brush for around 100k once. The item owner probably thought it was a plushie.
Then, I sold it for 650k, which is overpriced by a tad. Someone bought it and ta-da! I ended up with more Neopoints than I started out with!
5) Make your shop big and attractive. Check out different graphic sites
and find a nice shop layout that is clean and easy to read. After that, make it big
enough for lots of items! That way, the buyer will have a bigger selection.
Fill up your shop with your valuable items and then advertise! Advertise in
your guild, on websites (if they have a forum for Neopets Ads), at school (if your friends play Neopets as well), on the Neoboards, or wherever you see an opportunity. Nowadays, lots of sites have a "Neopet Advertisements" section for things just like shops or guilds and the such.
6) Ask your friends. If you are allowed to, ask guild members or Neofriends to buy things from your shop. If you have ever helped one of them out in reaching a goal, they will probably be very willing to help you out.
Now, I've told you all that I know about earning Neopoints. ^_^ Have fun and good luck!