I think that maximizing your Prolific income is WAY easier than maximizing your mturk income. So far as I can tell, there are only a few things you need to do, but you should do them all:
1) Always make sure you have answered all of the pre-screening questions. Don't forget to periodically go back to recheck them, since something might have changed. You might have gotten a new device, gotten a job, lost a job, etc. Always answer from the most optimistic perspective: for instance, if the question asks if you would be willing to record yourself via webcam, say yes. (Often I can't do this, since I am in a public place, and I'd probably refuse highly personal studies that involved webcam, but I have no problem in principle, and I have done such studies.) You can always decline a given study.
2) Use a page monitor. Some people use the Prolific Assistant extension (which for a time seemed to be broken, but I recently read that it was fixed). I currently use the Distill extension, but there are others. If you are checking more than once a minute, you will get errors, and Prolific states this directly, so you need to make sure you use a larger interval. Make sure the page monitor is always on. (Seriously. I just missed a full day since I had to turn mine off and forgot about it.)
3) If you see a study come up, go for it right away. (Unless, you can't, because you are liable to screw up a better paying job on mturk, or somewhere else.)
Prolific allows requesters to specify both a minimum and a maximum amount of prior studies completed by a participant, which is an option that mturk does not have. There are some reasons why a requester might wish to exclude people who have done many studies, but others just want to get the job done. The upshot is that you might seem to be doing well on Prolific, only to see your income fall off. But you can still make money there.
I can't say for sure if Prolific falls off in the summer, but I suspect not. The studies there cost more for researchers, so I think there are fewer grad and undergrad studies, and more studies by professors. But professors have more time in summer to actually do their research.