Forcing GiST index to be used

I have a custom datatype (the PostGIS geometry type), which I have
indexed using a GiST index.

The problem is, its difficult to get PostgreSQL to actually use the GiST
index.  The only way I can get it to be used is by 'set enable_seqscan =
off', which seems a bit cheezy.  What am I missing?  Do I have to make
some sort of amcostestimate() function or something?


thanks,
dave