I think everyone has pretty much mentioned everything on how to raise your bell value.
To start off with I did a lot of fishing, bug catching and such and sold them all once my museum had them. I was broke for a long time. Island bug catching is best if you remove all flowers and the trees from the middle part so the rare bugs only spawn. If anything that's not rare spawns, ensure you chase it away. The best time for this is after 11pm (you can always TT to that time if it's too late). You can earn around 300-400k in 30-45 minutes from doing this.
Other than that, having duplicates of anything spotlight, fortune cookie items, fossils, etc. and selling them on forums is better than selling them at re-tail as people needing those items are likely to pay more than you'd get at re-tail.
Moving in valuable villagers from the campsite will give you a lot of bells. Just do a little research and see how much they are and auction them off. My 1st camper was Beau who I moved in, then decided I didn't like him and let him go to the void without realising just how popular he was!
On some of the forums, some people hire people to landscape and pick weeds, etc. so you could look out for those and get hired and earn some extra bells too.
Buying quite a few turnips is always a good idea, but it's at a risk. But then again it's easier to sell by finding people that have great turnip prices on forums such as this and selling at a huge profit.
Once you build up a decent amount of bells, keep them in your bank account as you'll earn interest on them. The more bells you save the more interest you'll gain.
Hope that helps? Good luck.