This post describes how to install Debian Testing (current Testing is Wheezy). The procedure for installing Debian Stable (current Stable is Squeeze) is same, only the download links are different.
1. Install the necessary tools on your computer which runs a Linux as operating system. My ThinkPad runs Ubuntu 10.10:
$ sudo apt-get install mtools syslinux
2. Insert a USB stick into your computer, in most cases, your Linux should mount the USB disk automatically. If so, umount if firstly:
$ sudo umount /path/to/mount/point
3. Check the partitions information of the USB disk (assume the device is /dev/sdb):
$ sudo fdisk -l /dev/sdb
If the USB disk is never initialized and has no partition table, you need to create at least one partition on it.
4. Format (one partition is enough):
$ sudo mkdosfs /dev/sdb1
5. Install syslinux:
$ sudo syslinux /dev/sdb1
6. Mount the USB disk, and put necessary files into it:
$ sudo mount /dev/sdb1 /mnt
$ cd /mnt
$ sudo wget http://d-i.debian.org/daily-images/amd64/daily/hd-media/vmlinuz
$ sudo wget http://d-i.debian.org/daily-images/amd64/daily/hd-media/initrd.gz
$ sudo wget http://cdimage.debian.org/cdimage/daily-builds/daily/arch-latest/amd64/iso-cd/debian-testing-amd64-netinst.iso
Notes: Of course, since downloading images is time-consuming, you can download them separately and copy into the USB disk.
7. Create configuration file syslinux.cfg
$ cd /mnt
$ sudo vi syslinux.cfg
default vmlinuz append initrd=initrd.gz
Umount the USB disk, it’s ready for use then.
No CD/DVD, more green 
Have fun!