You can use maps that are created with maps.google.com.
There are three possibilities:
Use the parameter MSID from the url of the map at maps.google.com, see this article.
Use the link to map at maps.google.com and add at the end of the url: &oe=UTF8&output=kml. So: http://maps.google.co.uk/maps/ms?ie=UTF8&hl=en&vps=5&jsv=290a&msa=0& msid=102711323276911114625.00049400186fab69121bb&oe=UTF8&output=kml
Export the kml from maps.google.com and place it on your own website, see below.
Goto maps.google.com and select My Maps and follow these steps:
Click on link create new map (1) and make your own markers, polylines and polygons.
For using a map created by somebody else or a route created by maps.google.com:
Click on link import (2)
Place url in second field and add at the end &oe=UTF8&output=kml (3)
Press button upload (4)
Don't forget to save the map.
Click on link view in Google Earth (5)
Use save to disk (opslaan op schijf) and press ok.
Upload the file to you website and make sure it's reachable on the internet. Check it with maps.google.com, by placing the url to the kml-file in the search field and press enter.
Just use the kml-parameter to include the kml file. Now you can make changes at maps.google.com with your kml-file and the changes will show on your site!