It is entirely possible to multiclass to get good saves without suffering the penalty of lower BAB etc ... You just have to choose the right level for each class: even levels if your BAB is 1/2, level divisible by 4 if your BAB is 3/4. I can generate a character who has +16 base Reflex save at 20th level.
I think multiclassing becomes somewhat broken for saves and BAB with multiple classes. I've seen solutions discussed elsewhere, I forget who suggested these originally, but here's a solution that results in max. base save of +12 at 20th level, and slightly better BAB progression. If you implement these house rules, it will discourage multiclassing for the sake of getting good saves.
If you have a good save, your saves are (round down):
2+(good_save_levels*10/20)+((bad_save_levels+1)*6/20)
If you don't have a good save, your saves are (round down):
((bad_save_levels+1)*6/20)
BAB is simply (round down):
(good_BAB_levels*1)+(average_BAB_levels*0.75)+(poor_BAB_levels*0.5)
I put together a spreadsheet to make it easier for people. I've posted it here:
http://www.ocf.berkeley.edu/~chinnery/saving_throws_and_bab.xls.gz
It's a 5K gzipped Excel file.
Hope this helps.