I probably shouldn't admit this if I wanted to keep this "secret" to myself, but nah. I'll share it with you guys.
Buy turnips on Sunday. Price is irrelevant. Doesn't matter if it's even up to 110. Buy as many as possible. Wait until Wednesday. Go to the Train Station and Re-Tail boards here on TBT. Every single Wednesday there is at least 1 person with turnips selling for 500+ bells each. Because of the way turnip patterns are, bell prices tend to most of the time be their highest on Wednesdays, and someone here will get that bell boom high price. At least one. Usually several.
I didn't participate today because I was busy last weekend and didn't buy turnips. But the past 3 weeks I have done this and made millions of bells every single week even though prices in my town were always either terrible or with small profits.
Keep in mind that when you visit people with a high selling town, they will expect a tip. Sometimes it is mandatory, sometimes it is optional. People will ask different amounts too - sometimes they will leave it up to you, sometimes they will ask for a specific amount.
If you bought a lot of turnips, it's OK if you have to tip large amounts. I've been known to tip approximately 10% (most people I visit ask for either zero tip or far less than what I give them) as a thank you. Last time, that resulted in approximately a 500k (no, I didn't put too many zeros) tip for the person. However, I visited a town selling at over 600 bells per turnip. I made so much profit that it was worth it to give them the large tip.
But then again, I've found being very generous with people in these forums turns out in your favor. They will be generous back. I call it the TBT Karma.