- Don't work in a place that's really busy. If it becomes busy, I'll wrap up and move somewhere else.
- Buy something at least every 2 hours. If I've had too many coffees, then decaf at least, or a pastry. Spend around $5 each time.
- Tip! (Where appropriate). I like to add $1 for each purchase.
- Get to know the employees. Learn their names and greet them.
Yes, it is a bit more expensive (usually ~20$ for a day), but you get the same "coffee shop experience" and don't have to feel guilty or obliged to eat/drink (often not so healthy) stuff all the time.
Once you've worked from a few coffeeshops you can recognize the places that welcome remote workers - they go out of their way to provide lots of outlets and lots of 1/2 person tables. If a place has bright lighting, loud music, very few outlets - it's polite to find somewhere else.
Also, the amount to spend can scale with your budget; it seems more important to just to get up and buy something, just so you don't appear completely cut off from the rest of the world.
I normally get espresso drinks, but sometimes I'll get a normal drip coffee, which usually has a cheaper refill ($3 for the first, and $1 for a refill), and I'll tip on each refill.