Use different operators to for better search results. Common characters are : and + and - for example

:category1,category2 - search in category1 and category2 (use underscore instead of space in category names)

:category1,category2 keyword - search in category1 and category2, search by keyword

+keyword - must contain keyword

-keyword - must NOT contain keyword

>keyword - give more importance to keyword

<keyword - give less importance to keyword