Do not be over-generous when tipping or paying for small errands or favours; this will give out the wrong signals.
Be wary of well-worn "sob" stories, eg "No milk for the twins"; "I live in Brikama [or Lamin], just around the corner". The list goes on and on.
If you are walking along the beach when approached, it helps if you walk into the sea. Gambians are keen footballers and are invariably reluctant to get their trainers wet.