Valsalva Maneuver has been a traditional initial treatment for SVT. However historically, it has had a low success rate. 
A recent article in Lancet (Lancet 2015;386(10005):1747) demonstrated a modified maneuver that doubles the success rate.
Below is a You Tube video demonstrating the technique.
If unsuccessful the first time it can be repeated x 1.

Secondly, when giving adenosine, we have used a 3 way stop cock to inject the adenosine followed by a 20 cc Normal Saline Flush. It has been shown that simply drawing up the adenosine in a 20 cc N/S flush syringe and giving it all as a 'push', works just as well and is much quicker.