I have "easy mailers" and "tough mailers" that I use. The ones that I call easy are usually ones where I'm upgraded and have a lot of referrals. They're quick in and out mailers, and I use them most often. Then the ones I consider to be tough, I have to work for credits, but I expect and normally achieve better results with these. Often reserved for special occasions
LFMVMS are quite easy to earn credits in because you can just open a lot of links at one time from the site itself, but mails that need to be clicked from the inbox, I use the vertical split pane on my individual folders so that the next mail opens automatically. Something I've been doing for ages, so I assume its not great secret and most others are too 