When travelling in Samoa you will find people address you more respectfully when you dress properly. As a general rule of thumb, keep your shoulders covered, wear clothes that are not skin-tight and pants/ skirt that are close to knee length. This dress code is not always necessary, but certainly never wrong.
In Samoa, shorts are fine (better if they go to the knee) as long as they aren’t too skimpy. The same goes for tops, skirts and dresses. Many Samoans were lavalavas (sarongs) in their day-to-day lives and they can be fairly revealing but they are also traditional dress.
Bikinis are fine (again not too revealing) on tourist beaches such as the ones that belong to the resorts. Samoans tend to swim totally clothed but tourists are not expected to follow suit. Be tasteful and wear swimwear that is not too revealing. Swimwear should be worn only at the beach. If you are driving/travelling around on your own (in a group) and want to stop at a village beach shorts and a t-shirt for swimming are preferable. Bear in mind when swimming on a village beach you will probably be asked to pay a small fee as the beaches are also cultural land and if you aren’t appropriately dressed, you may be asked to leave. Swimming on a Sunday (except at resorts) is taboo. Always ask before swimming on a Sunday. You may be asked to pay a fee to swim, hike or enter private land. Please remember that all land in Samoa is private, including beaches. Those who live there have a right to a small fee for you to use their land, and for some, that is their only income.
If you do attend a church service, pants, a dress or skirt are preferable that cover the shoulders and do not go above the knee. Samoa is a fairly religious country and in some ways is traditional, but it is changing fast. A few years ago, it was frowned upon for a couple to walk hand in hand around town or to display any sort of affection but you see this occurring everywhere in Samoa now.