I measured myself using guidelines from a bra intervention thread and settled on a 38F as my size. I bought some bras in this size last year and thought they were an OK fit but gradually felt they weren't great.
I went bra shopping (was limited to M&S that day) and couldn't find a single comfortable bra.
I've been browsing online and had a look at Figleaves today and saw this
I found it very useful to see the issues with pictures etc. They sized me at a 36GG which is close to what someone recommended to me after I started a thread about my unsuccessful shopping experience.
I think it's worth a look.