My tip :)
I gain a few clients every month. I have not submitted a client to any search engine apart from:
c) Inktomi PFI
d) Teoma PFI
e) FAST PFI
f) Lú UK
Then I do it once and forget about it (apart from re-subscribing).
For those client who don't take PFI, I get a DMOZ link - and then forget about it.
My site hasn't been submitted to a search engine since early 2000.
Bottom line - if you get a link from a site which is already in a major SE, you don't need to submit. You WILL get crawled. Directories (apart from DMOZ) usually need paying - and even if they don't, how would you feel if you were an editor getting the same site submitting again and again and again and again.....
The submit often or you will fall off argument is rot!