With the stable climate on Lombok, you do not have to worry about what to wear. Airy, summer clothing is the most comfortable.
Only during official occasions or when visiting the authorities you will need to wear a polo, shirt and long trousers. For all other occasions summer/holiday clothing will suffice.
Since Lombok is mainly Muslim, you will need to take into account that in many areas outside Sengiggi, Mataram or the main tourist attractions, covering the shoulders of women is very much appreciated. Also covering the knees with with 3/4 or long pants or a skirt is recommended to respect the local culture.
Wearing a two-piece or bikini at the beach is not a problem. Make sure you wear something on top when you leave the beach. Topless sunbathing is not accepted.
During the months July and August it can feel a bit chilly in the evenings. A shirt with long sleeves will be enough to make you comfortable.
Please make sure you always wear sun protection or cover your skin to avoid sunburn. Because of the light breeze, the sun can be treacherous!