It seems like they release another batch of memes every hour, on the hour, and I believe they order memes in the Latest stream in order based on when they are generated (not on when they are submitted). So it makes sense to submit a meme pretty close to when you make it. If you are thinking of submitting a meme that you made a while back (e.g. a meme comment on someone else's meme, or something you never got around to submitting), then it makes sense to remake it before submitting it so it doesn't come out on the very bottom of the batch.
When I can, I try to make my memes around the 45 or 50 minute mark, in the hopes that that will help them be nearer to the top of the pile. Of course, if it's a new image you're uploading to imgflip for a meme, all bets are off - those sometimes take hours and hours to come out and you don't know where they'll be. At least I don't.
Feel free to use some of those tips if it helps. Hope you don't mind the comment! :)