These methods should increase the lifespan of the SD card by minimising the number of read/writes in various ways:

Disable Swap

Swapping is the process of using part of the SD card as volatile memory. This will increase the amount of RAM available, but it will result in a high number of read/writes. It is unlikely to increase performance significantly.

Disable swap with the swapoff command:

sudo swapoff --all

You must also prevent it from coming back after a reboot:

For Raspbian which uses dphys-swapfile to manage a swap file (instead of a "normal" swap partition) you can simply sudo apt-get remove dphys-swapfile to remove it permanently. Best to remove because setting the CONF_SWAPSIZE to 0, as explained in this answer, doesn't seem to work and still creates a 100MB swap file after reboot.

to remove it permanently. Best to remove because setting the to 0, as explained in this answer, doesn't seem to work and still creates a 100MB swap file after reboot. For other distributions that use a swap partition instead of a swap file, remove the appropriate line from /etc/fstab

Disabling Journaling on the Filesystem

Using a journaling filesystem such as ext3 or ext4 WITHOUT a journal is an option to decrease read/writes. The obvious drawback of using a filesystem with journaling disabled is data loss as a result of an ungraceful dismount (i.e. post power failure, kernel lockup, etc.).

You can disable journaling on ext3 by mounting it as ext2 .

You can disable journaling on ext4 on an unmounted drive like this:

tune4fs -O ^has_journal /dev/sdaX e4fsck –f /dev/sdaX sudo reboot

The noatime Mount Flag

Assign the noatime mount flag to partitions residing on the SD card by adding it to the options section of the partition in /etc/fstab .

Reading accesses to the file system will no longer result in an update to the atime information associated with the file. The importance of the noatime setting is that it eliminates the need by the system to make writes to the file system for files which are simply being read. Since writes can be somewhat expensive as mentioned in previous section, this can result in measurable performance gains. Note that the write time information to a file will continue to be updated anytime the file is written to with this option enabled.

Directories in RAM

Highly used directories such as /var/tmp/ and possibly /var/log can be relocated to RAM in /etc/fstab like this:

tmpfs /var/tmp tmpfs nodev,nosuid,size=50M 0 0

This will allow /var/tmp to use 50MB of RAM as disk space. The only issue with doing this is that any drives mounted in RAM will not persist past a reboot. Thus if you mount /var/log and your system encounters an error that causes it to reboot, you will not be able to find out why.

Directories in external Hard Disk

You can also mount some directories on a persistent USB hard disk. More details of this can be found in this question.

The Raspberry Pi can also boot it's root partition from an external drive. This could be via USB or Ethernet and means that the SD card will only be used to delegate to different device during boot. This requires a bit of kernel hacking to accomplish, as I don't think the default kernel supports USB storage. You can find more information at this question, or this external blog post.