Steam or dry heat.
The advantage of steam is you can't get it too hot. The disadvantage is it cools so quick, it's best if you can make a jig that steams it while you are making the bend.
I've never actually boiled.
Dry heat is fine but with Yew the sapwood needs protecting.
There are a lot of posts about both on my Bowyers Diary (the search facility on the blog works better than most)
It really depends what you are doing. I use a 5litre plastic bottle to confine the steam around a small part of the bow for localized bending, the steam is generated using a wallpaper steamer. This allows you to apply the bending force while the steam is on the wood.
This post is a good one:-
http://bowyersdiary.blogspot.co.uk/2012/02/spot-of-steam-bending.htmlIt shows the bending force is applied by a brick tied to the end of the bow! As the steam takes effect, the brick lowly pulled it down.
Del