I'm very fortunate in that one of my early subscribers started advertising when I first offered it and has been an advertiser ever since, so I really don't have to work for it. She buys the top position every issue, usually months in advance. The other two positions I either sell or I experiment with, usually one of each. I may trade one spot for a reciprocal ad in other ezines or try an affiliate program there. I've never had to look for advertisers, they find me.
Word of mouth is the best advertising after all.
I don't want this to sound like bragging, but it's my unique style and humor that people enjoy, and is why my turnover rate is extremely low. I only had one person unsubscribe during my first year, and mom swore it was accidental. I get idea's everywhere. On my computer I just quickly type a reminder in Notepad. If I'm not at my computer I write it down on a scrap of paper.