I tend to look at this much as I look at other storage solutions.
In the past, "back in the day" on my computer I would download tons of movies, music, etc, and store them on my computer. Then I'd run out of space (whether I had 100 GB HDD or 500 GB HDD, didn't matter), and I'd go out and buy an external hard drive and fill that up. Then I'd buy another, and another. Then I'd never look on the drives, never be able to find what I was looking for, etc.
Eventually I started uploading all of my images to Flickr when I was done sorting/processing them and then deleting the originals (I do archive super important photos and negative scans). I delete movies once I've watched them because I'm never going to realistically watch them again. I rate my music 1-5 stars, delete anything that's less then 3.
Now, the same basic ideas I now apply to my iPad and iPod Touch (I'm an Android person as far as my phone goes, for cheaper data and free tethering).
I have a 16GB iPad, and I only sync my 5 star music (songs I'm really going to want to listen to). I manually add movies if I want to watch them, and then delete them when I'm done. I've purchased a TON of apps, but only keep the ones that I find myself using regularly, which for me means things like Mint, iMovie, GoodReader, It's Playing, and maybe 10-15 other applications. If I buy a game I like then I normally play that game for a week, or a month, until eventually I beat it or get bored of it, then I delete and move onto whatever is new. I also have netflix which I use to watch a lot of video content.
Now of course if you consume media different then I do then you might have more problems, but I'd try to move towards this direction as I find it more sustainable then solving the problem by purchasing larger storage space. Leverage the cloud (feel so stupid saying that) as you can (another example of this is that I upload videos that I've taken to YouTube directly from my iPad/iPod Touch, then delete them. If it's an absolutely critical video this might not work for you, but for most videos YouTube is 'good enough" to store them).
The one thing I can see bothering me is video - if I actually take many 1080p videos, then I'll probably need a fair amount of empty space just to allow me to record 10-30 minutes of video without issue (not to mention processing with iMovie and saving). With my iPad 2 I found it pretty easy to stay around 50% capacity though, so I'm guessing with the iPad 3rd generation it will not be too impossible.