Firstly, a diaphragm does not kill sperm, it simply helps prevent it from entering the cervix.
Secondly, if you plan on engaging in sexual activities with more than 1 person, a condom would be a good idea, otherwise its not so neccessary.
As for advice goes, there is one main pointer that all girls should heed:
Keep it wet! lots and lots of saliva is good, helps reduce friction and makes it feel much nicer!
Also, be enthusiastic, dont just bob up and down on it, use different movements and pressures, but most important of all -
ASK HIM WHAT HE LIKES! You can be any man's best blow if you know exactly what he likes and doesnt!
[URL=http://homepage.mac.com/durf/ot-faces/faces/Jenny.jpg]Me and my Lady[/URL] | {1y 10m 7d}
"An eye for an eye makes the whole world blind" - Mahatma Gandhi