Here's a tip I was taught when shooting a rifle but it works equally well (for me anyway) when I need to hold a camera steady for a shot. It's all about how to hold your breath. Most people breathe in and then hold their breath while taking the shot. But this means you are putting your rib muscles under strain and could cause a bit of trembling. Try filling your lungs then breathing out about a third of the way, then stop breathing out while you take the shot. I find this useful for those times when a tripod/monopod isn't being used.