Limit Joins (or Plain Improve Qry)

Hi,

I got a qry that takes 10s. This is off course too long.
As I described in my earlier posts, I'm no sql hero, but I'm on a neverending quest to improve.
Currently I need a select statement over several (7) tables and return fields from almost all of them.
Furthermore I have optional restrictions on fields spread over all those tables, based on a set of parameters.
What I initially do is inner join all my tables and then have a big where clause that adds where-rules depending on the arguments given.

Like so:
select c.clr_id, n.clr_nm, co.long_nm, p.paint_cd, a.appl_desc, m.modl_nm, y.yr_num
from CLR c
inner join paint_cd p
on c.clr_id = p.clr_id
inner join clr_use_yr y
on y.clr_id = c.clr_id
inner join co
on co.co_id = c.co_id
inner join modl m
on y.modl_id = m.modl_id
inner join clr_appl ca
on c.clr_id = ca.clr_id
inner join appl a
on ca.appl_cd = a.appl_cd
inner join clr_nm n
on c.clr_id = n.clr_id
WHERE
(CASE @regn when '' THEN '' ELSE n.regn_cd END) like (CASE @regn when '' THEN '' ELSE @regn END)
AND (CASE @co when '' THEN '' ELSE co.long_nm END) like (CASE @co when '' THEN '' ELSE @co END)
AND (CASE @clrNm when '' THEN '' ELSE n.clr_nm END) like (CASE @clrNm when '' THEN '' ELSE @clrNm END)
AND (CASE @paintCd when '' THEN '' ELSE p.paint_cd END) like (CASE @paintCd when '' THEN '' ELSE @paintCd END)
AND (CASE @applDc when '' THEN '' ELSE a.appl_desc END) like (CASE @applDc when '' THEN '' ELSE @applDc END)
AND (CASE @useYear when '' THEN '' ELSE cast(y.yr_num as varchar(4)) end) like (CASE @useYear when '' THEN '' else @useYear END)
AND (CASE @modlNm when '' THEN '' ELSE m.modl_Nm END) like (CASE @modlNm when '' THEN '' ELSE @modlNm END)

It appeared to me that by getting those where clauses in the join, I would limit the calculations. So the joins where similar to this:
inner join paint_cd p
on c.clr_id = p.clr_id
AND (CASE @paintCd when '' THEN '' ELSE p.paint_cd END) like (CASE @paintCd when '' THEN '' ELSE @paintCd END)

The result though, was exactly the same.
So, where in my thinking am I wrong and what is the correct way to go about this?