AS prepending doesn't really "force", it attempts to influence inbound eBGP traffic by having traffic see a preferred path with a shorter AS number of hops than another path. I.e. what's normally done, your AS is prepended numerous times to the less preferred path's eBGP neighbor.
Outbound path selection is more often influenced (locally) by other BGP attributes (that have path selection priority over the number of AS hops).
AS prepending is usually done by the customer, as an ISP is more likely to use other BGP attributes to override AS path selection.
Understand, at the customer edge, you normally have full control deciding what your initial outbound next hop will be. It's attempting to influence and/or control input traffic path selection, that can be complex.
A couple other ways of managing inbound path selection, besides AS prepending, would be to share some route preference policy with your eBGP peers, often done using community strings. Another method to influence input path selection is by advertising different prefixes to your BGP neighbors. For example you might split a /23 into two /24s, but only one /24 is advertised to each BGP neighbor while the /23 might be also advertised to both.
