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!  (EC2 pricing)

I used to have one small instance $73 / month.  Now a micro instance goes for $6.43 / month!

		$/hour		$/year(s)		$/month	Notes
Unreserved		$.065	.065*24*365	$569.4	/12	$47.45	Dec. 2012
1 year small reserved	$195	$.016	+(.016*24*365)	$335.2	/12	$27.93
3 years small reserved	$300	$.013	+(.013*24*365)*3	$413.88	/36	$17.82
1 year micro reserved	$62	$.005	+(.005*24*365)	105.80	/12	$8.82
3 years micro reserved	$100	$.005	+(.005*24*365)*3	$231.4	/36	$6.43
Unreserved		$.085	.085*24*365	$744.60	/12	$62.05	Jan. 2010
1 year reserved	$227	$.03	+(.03*24*365)	$489.8	/12	$40.82
3 years reserved	$350	$.03	+(.03*24*365)*3	$1138.40	/36	$31.62
Unreserved		$.10	.10*24*365	$876	/12	$73.00	Mar. 2009
1 year reserved	$325	$.03	+(.03*24*365)	$587.8	/12	$48.98
3 years reserved	$500	$.03	+(.03*24*365)*3	$1288.40	/36	$35.79

 

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.