When learning a rhythm for a song with a complex strumming pattern, you shouldn't try try to learn it by up/down strokes by heart. The reason is that just won't sound very good, because there's only so much you can remember, before it becomes a one big mess of ups and downs. So you will probably end up making it more simple to be able to remember it while playing, which won't give you satisfactory results.
If you've just started to learn a song, you shouldn't try to learn it 100% correctly at once, at least that's what I think. You should divide the task into pieces. First, be sure you you're comfortable with all the chords and changes(they ring out clearly), then learn to play the song through with a fairly easy strumming pattern. When you're entirely familiar with the structure of the song and all, you can try to learn to make it more complex. Listen to the record and try to connect different rhythm parts with the lyrics, e.g. when a certain line is said, something interesting happens on the rhythm, or something. Like somebody said, you have to feel the song, try to hear it inside your head while playing it, rather than learning it from paper.
Also, try to avoid being repetitive. Well, there is certain basic thing going on in the verses and chorus, etc, but don't play the exact same thing every time you play a certain part, because it becomes boring very soon. Try to add something here and there to make it a bit more diverse, so that people can't be totally sure of what comes next. Anyway, be patient with it and it'll come eventually.
I don't know if this answered to the question, but I hope that at least somebody found this helpful!
PS: this is a second time I write this message. On the first time, after I had pushed the submit button, I had to log in again and the message was gone!

Why the fuck was that?? I almost threw my pc through the window!
