You can reserve Amazon EC2 instances (see announcement).  They're called reserved instances because you're reserving an instance so you know it will be available if/when you need it.  But, if you know you'll be using an instance for a year, or three, it's also much cheaper!

I used to have one small instance running at $.10/hour ($73/month).  (As of Jan 2010, it's $.085/hour or $62/month).  Now with a reserved instance this fee goes down to $49 $41/month for a year, or $36 $32/month for 3 years!

Here's the commands to create a reserved instance (you'll need the new Amazon EC2 API Tools with the new commands):

• Find current instance type:

        $ ec2-describe-instances
            RESERVATION    r-........    ............    default
            INSTANCE    i-.....   ec2-..-...compute-1.amazonaws.com
            domU-..-...compute-1.internal    running    0
            m1.small    2009-01-20T16:21:12+0000    us-east-1a

• From above:

        zone: us-east-1a
        type:  m1.small

• Find reserved instance to buy (using above zone and instance type):

        $ ec2-describe-reserved-instances-offerings -z us-east-1a -t m1.small
        OFFERING    248e7b7..... us-east-1a    m1.small    3y     500.0    0.03    Linux/UNIX
        OFFERING    4b2293b..... us-east-1a    m1.small    1y     325.0    0.03    Linux/UNIX

• Buy one reserved instance using entire offering id from above (3y in this case).  This command costs $:

        $ ec2-purchase-reserved-instances-offering -v -o 248e7b7..... -c 1
            RESERVEDINSTANCES    f127bd27.....
            REQUEST ID    ........-....-....-....-............

• Look at purchased reserved instance:

        $ ec2-describe-reserved-instances
            RESERVEDINSTANCES    f127bd27.....    us-east-1a    m1.small    Linux/UNIX    3y     0.03    500.0    1    2009-03-13T16:01:39+0000    payment-pending

• You should start getting charged the lower rate right away, although it doesn't show up on Amazon's site right away.  You don't need to restart your existing image.