It looks like securely erasing an SSD is a different chore than erasing a traditional hard drive. Unconfirmed, but it looks like Apple is actively disabling the legacy erase techniques for SSD drives, since they don't work as noted in Ask Ars: How can I securely erase the data from my SSD drive?
As pointed out in a recent research article, there isn't a standard method for securely deleting data from a solid state drive. Hard disk drives have had this problem solved for ages, and can execute a secure delete by filling the space occupied by an incriminating file with zeroes or multiple writes of different characters...
The overwriting procedure that works so well for HDDs doesn't work as well for SSDs for a couple of reasons. One is that many SSDs have extra storage space that's not accessible by users. This is called over-provisioning, and some deletion tools won't give you access to this area (if you can go through the BIOS and uninstall your drive, you can get full access). While the research paper noted above that overwriting did obscure all of the data in some of the SSDs they tested, some still had old readable data on them after twenty overwrites. This can happen because of firmware bugs, and unless you're able to physically confirm that this procedure works on your SSD of choice, it's not reliable enough for a real secure erase.
Likewise, it is almost impossible to securely delete an individual file on an SSD, because the way that SSDs write and delete files is scattered, and a user has no control over what an SSD is doing where. If that's the kind of security you're looking for, your best bet is encryption, which we will cover in a little bit.
Many SSDs today also come preinstalled with secure wiping utilities that are meant to actually eradicate data from cells. But of the twelve drives tested by those researchers, only four of twelve erased their data reliably, and one that claimed to be securely erasing everything was just doing the old "forget where everything is and leave it there for now" trick, and all the data was still retrievable. Some drives are known to have better erase tools than others—for instance, drives with Indilinx controllers have a "Sanitary Erase" that deletes all data and restores drives to their out-of-the box condition.
Your best bet going forward is to use encryption and then throw away the encryption key to "securely erase" it. So going forward, if you are concerned with security and you're using SSD, you may want to enable FileVault2 immediately before you start putting data on the SSD. Unfortunately, for data that's already on the drive, it may be extremely difficult to securely delete it, depending on the SSD used in the MacBook Air. There maybe be a reset tool for the SSD from the manufacturer, but seeing how it's an Apple OEM specific part from a 3rd party vendor, it's probably going to be unlikely to exist.
If security is an issue, you may want to hang onto the SSD or destroy the SSD to be the most secure.
If your MacBook Air supports Lion Recovery Mode, and you're using full disk encryption such as FileVault2, you will not need anything else to do it other than following the steps below.
Boot your MacBook Air into Recovery Mode by holding down Command+R while booting it.
Open the Disk Utility Program, select the drive you want to securely erase.
Select the "Erase" tab.
Click "Security Options..."
Drag the slider to the Most Secure setting or somewhere in between and then select "OK".
Then click "Erase..." and follow the remaining on-screen prompts.
Note: For more information on how secure the Disk Utilities Secure Erase feature is, see About Disk Utility's erase free space feature