Dual layer dvd - Use DVD Shrink to rip/rencode then use CopyToDVD to write those files back to a blank DVD.
Single layer dvd - Use DVD Decrypter to rip it to a single ISO image file, use Nero to burn that ISO to a blank DVD.
When using DVD Shrink, I always take out all the subtitles since they're rarely if ever used, take out any additional languages. I raise the compression on the menus and extras all the way to level 10, then just raise the main movie compression as needed until it will fit on a single DVD. Also, do not get the newest version of DVD Shrink. It's got some problems with is decrypting strings and has a hard time ripping the movies. You want to get the older version, 2.3. I've went as high as level 9 on the compression on the main movie and notice no quality degradation on a friends home theater with 8ft screen.
Dual layer burners are slated to hit the market in the US in about 2 months, one by Sony, one by Philips. They're expected to retail for about $230-$250. However, don't jump into one yet because these first ones will only use their own media, i.e Sony media in the Sony drive and the blank dual layer DVD's are expected to cost about $5 each for now. I'll wait until the format is standardized a little more and the media prices come down.