I was going to suggest shred, I can't find a better way to do it "via software".
Probably if you need further security you will need to buy specific hardware or "one-use hardware" which you will wipe out with a professional demagnetizer.
Some more information I read a while ago about secure deletion using shred can be found in this article
Let me quote some info:
How many passes?
You can go with the defaults or set your own. The US Government
standard is seven passes but there are apocryphal tales of files being
recovered after fourteen passes. Shred's default is twenty five but
secure-delete, below, uses thirty eight.
Put shred on steroids
Shred is a powerful command for sure but it doesn't cover all the
angles. For that you need something even more powerful; here Ubuntu
users are in luck because they have at their disposal a tool that can
deal with data in RAM, free space and in swap. Other distros can
download the tarball. Just apt-get install secure-delete in a console
(as root--su) and issue any of the following commands:
srm confidential.txt (securely deletes files and directories) smem
(wipes data from memory to combat data remanence) sfill mountpoint/
(wipes the free space on a disk) Use with a live CD, possibly as root
sswap (wipes swap partitions used when RAM is full. Use in conjunction
with smem) The last command requires you to turn off swap first. Just
open /etc/fstab or type cat /proc/swaps to see where swap is mounted
and then disable it with sudo swapoff/dev/hda2 (insert your own detail
here). Swap can now be wiped with sudo sswap /dev/hda2 and swap
re-enabled again with sudo swapon/dev/hda2.
Also you can read the epilogue and recommendation in this paper
There are two ways that you can delete data from magnetic media, using
software or by physically destroying the media. For the software-only
option, to delete individual files under Windows I use Eraser and
under Linux I use shred, which is included in the GNU coreutils and is
therefore in pretty much every Linux distro. To erase entire drives I
use DBAN, which allows you to create a bootable CD/DVD running a
stripped-down Linux kernel from which you can erase pretty much any
media. All of these applications are free and open-source/GPLed,
there's no need to pay for commercial equivalents when you've got
these available, and they're as good as or better than many commercial
apps that I've seen. To erase SSDs.... well, you're on your own there.
For the physical-destruction option there's only one product available
(unless you want to spend a fortune on something like a hammer mill),
but fortunately it's both well-designed and inexpensive. DiskStroyer
is a set of hardware tools that lets you both magnetically and
physically destroy data on hard drives, leaving behind nothing more
than polished metal platters. It's been carefully thought out and put
together, there's everything you need included, down to safety glasses
for when you're disassembling the drive. It's had very positive
reviews from its users. If you really want to make sure that your
data's gone, this one gets my thumbs-up (and this isn't a paid
endorsement, if only other technical products had this level of
thought put into the workflow and usability aspects).
I don't really believe you can restore the information after 32 passes, but if it is possible I think you can be sure that it will need a very special and sensitive machine not available for normal humans.
Also, what @LucasKauffman said =), beware with SSD drives.