The best way to accomplish this is to use an "include" statement in your SPF record. This way, if we ever change our IP addresses for any reason, you won't need to update any of your SPF settings. This is the same thing google does. Here is an example from our hq.bill.com spf record, of how it will look:
[dpeters@web1 ~]$ dig +noall +answer txt hq.bill.com
hq.bill.com;60ITX"v=spf1 ip4:184.108.40.206 include:_spf.bill.coinclude:_spf.google.com ~all"
Be sure to include the following in your SPF record from the above example:
(Please note that there is an underscore in the hostname, and there is no space between the colon and the underscore.)