It comes down to Club momentum. When you are winning, it's there and folk will flock to join - look at Balmain & even Blue Mountains. When you are losing, the same folk who joined late will leave early. After a couple of years winning you become Strong - and that brings it's own momentum & rewards (look at Colleagues and Drummoyne especially in their lower grades).
Alternatively, talking with some blokes at Blueys (2yrs ago) and even Hunters (this year) who went "up", they found it hard to keep up the flow of players to replace the injured and buggered when playing harder/faster footy consistently every weekend.
What I have learnt over the years is that come July you need to start sand-bagging your gun/senior players to identify holes and begin to fill for next year (either promote those untested and/or begin the search for replacement). If you leave it later then that you rely on whatever random shows up in pre-season which is too "hit & miss".
How do you search and recruit replacements? Mostly word of mouth, there's a couple of websites and having a benefactor or three who can help with a short term job, cheap rent/spare room or signature on a visa whilst a guy finds his feet is what is most valuable. I've played for a few clubs who have solid contacts with country clubs (steady feed of players who can work and also workers who can play) but also good relations with some foreign clubs so back-packers & even some professionals know where they can find a friend/bed/job in Sydney/Limmerick/Swansea/Glasgow is sound. I've trod that path and it was fantastic. That connection gets built over time but is well worth it.