Tips have to be earned not expected.
In the hotel: room boy/girl LKR 500/week, pool boy LKR 500/week, bell boy/porter LKR 50-100 depending on how far he has to carry your luggage.
Restaurants: if 10% service is not already added to the bill a 10% tip is reasonable. If 10% service charge is added usually tip less than LKR 100.
Driver/guide: LKR 300-500/day.
If you visit temples that don't charge entrance fees a small donation (< LKR 500) is always welcome.
Tips are always down to discretion.