The MED is used between eBGP neighbors and is the only means you have to try and influence how a remote AS views your routes.
To influence how a remote AS selects the best path for your given network you define an ACL to permit the routes you wish to influence, next you confiigure a route-map which matches the ACL then set the Metric to your value (remember lowest wins) finally you apply the route-map to your neighbour statement under the BGP process.
R1(config)#access-list 10 permit 192.168.10.0 255.255.255.0
R1(config)#route-map MED_OUTBOUND permit 10
R1(config-route-map)#match ip address 10
R1(config-route-map)#set Metric 100
R1(config)#route-map MED_OUTBOUND permit 20
R1(config-route-map)#set Metric 200
R1(config)#router bgp 500
R1(config-router)#neighbor 10.1.1.1 route-map MED_OUTBOUND out*
* route-map applies outbound to the routes you are advertising in BGP that match the ACL you configured.
In the example above routes for 192.168.10.0 advertised to BGP neighbor 10.1.1.1 will have a MED or Metric of 100. The default for MED is '0'
The next line (permit 20) states that 'All other routes' will have a Metric of 200. Remember Lowest Metric wins so by setting 'All other routes' to have a MED of 200 you are trying to influence the remote AS to use an different router (say R2) for 'All other routes'.
To ensure you influence the remote AS to direct traffic for 192.168.10.0 to R1 set the MED for 192.168.10.0 on R2 (plus any other routers that have a vaild path to 192.168.10.0) to a figure greater than that set on R1. Also for 'All other routes' set the MED to below that of the R1 catch all statement.
Keep in mind that MED or Metric is meets criteria 6 in the list of match statements for BGP path selection. If your route matches any of the preceeding criterion for Best Path selection then MED will not influence how the remote AS views the route you are advertising.