Ensure that people know what they will get by scanning the QR code. Use phrases like “Scan to donate” or “Go to Givealittle” to show what the QR code does. e.g.
Go large
Make sure the QR code is large enough to be easily scanned by a smartphones from a meter or so away, so people don't have to work too hard to use it.
Get it out there
Put the QR code in a location where your target audience is likely to see it. Consider high-traffic areas, relevant print materials, or get creative… t-shirts, cars…