how to just back to previous location quickly?

[wang yin]

Hi,

I use M-. to go to a tag. And I M-. to some other places...
And I can M-* to get back to where I was step by step.

But how can I get forth then? 
I mean, if I jump to other locations with grep, tag, ... How can I
quickly get back to where I was? bookmarks are too slow.

Thanks.

-- 
Wang Yin
DA Lab, Tsinghua University,
100084
Beijing China