I use a combination of strategies. I need to take breaks frequently during the day and when I do, I check some job boards and check the forum. I have done this long enough I know roughly when who is likely to post the shops I want. My bookmarks folders in Firefox include one of my current 'best' companies. Those will get checked several times a day. I then have other MSPs broken down into manageable size bookmarks and I will generally make it through the more promising ones of those weekly out to companies I have been registered forever with and never found a shop, which get checked once every blue moon.
I found the X shop posted today, today is 8/30, I will mark my shop sheet for 9/28 to start checking for that shop, unless, of course it is my company that posts late Saturday nights and I will mark the Saturday before 9/30 to watch for the shop. Every other Thursday is marked to watch for my shops from another company. All of this is kept on my shop sheet and the recheck dates just get cut and pasted into the future sections of the sheet once the shops post.
40 companies is not a lot. My 'best' companies tab has 20 and that changes each year as clients move or new clients enter the system. If you want more work you need to register with more companies. And certainly some you will never see work from, but you cannot know in advance because they may have clients everywhere in the country except where you are.