top ways to decrease weight

i have a specialized hard rock mt bike from way back in 1995, and would
like to make some inexpensive changes to trim down its rock-like
weight. what would give me the most bang for the buck? i do plan on
switching to slicks for city (and trainer) use. as far as what
inexpensive means ... i'm trying to do all this for about $100-150
(probably mostly used components).

thanks!