There are usually as many answers to that question as there are fly fishers.
Personally, I have a tech vest - two pods on the front and one on the back - but I use a lanyard for the quick trips where I know exactly what to throw at them.
A lot of guys prefer to have everything right there when they need it. I've found that I need a fly box or two, forceps, nips, floatant, tippet, and a net. Past that, it's just extra stuff i might need.