It's sometimes difficult to determine whether poor images are due to camera shake/ subject movement or down to poor focus. Try taking photos of an inanimate object with the self-timer function to remove the possibility of movememnt affecting the shot.
Remember that the images will not appear on your monitor as sharp as the images shown by experienced digiscopers on the Internet, the images always require further in-computer sharpening and often some levels adjustment as well.
Even the most successful digiscopers have a relatively high failure rate with their shots. Although it varies slighlty, 90% of shots will be too poor to print at above 10x8 inch. Take as many shots as you can of the bird, if you are presented with a good opportunity.... make the most of it with 50+ photos.
We all go through bad patches where every shot turns out poor, leading to disillusionment with the method. Keep at it.
Taking the Photos
First off, I am one of the many digiscopers who favours focusing the spotting scope on to the subject with the digital camera already attached, therefore using the monitor of the camera to judge when the subject is in correct focus. This method is more akin to standard photography with a traditonal slr camera and lens.
One of the golden rules with all photography is that the camera doesn't always see things as the human eye does, so focusing with the image the camera is seeing from the scope usually provides the best results.
When you first attach your digital camera to the scope, don't be tempted to go straight out and start taking photographs of birds, it's far better to stick to your backyard and practise on an inanimate object placed at 20-30 metres/yards away from where you are photographin from. Being in close proximity to your computer will make life far easier as you will be able to check your results on a large screen rather than the tiny camera monitor.
When you are happy with the results from your trials, move on to birds that are relatively static to gain confidence.
Whether you are using a cable-release, Nikon remote or just your finger to trigger the shutter, do not press the button straight down in one movement. Half press to get a focus lock on the subject, then gently press further to actually take the shot. You may even wish to fine tune the scope's focus whilst holding the shutter button at half-press, though it's unlikely you will have time with most birds.
As with all wildlife photography, it is very important to know your subject. Careful observation of the bird will allow you to know it's favoured perches, therefore you can wait for the bird rather than chasing it around with the scope. Even with the magnification that digiscoping offers you should try to get as close as possible to the subject, laying in wait is by far the best approach and this may entail a portable hide/blind.
For birds that prefer to be on the ground, try to get your spotting scope as low as possible to the ground so as to give a far more photogenic angle and also make the bird stand out against a distant background rather than the ground a few inches away from it, this will help your camera's auto-focus to lock on to the subject. In addition, having your tripod as low as possible (with legs spread out, if possible) will do wonders for stability.
You may think the sun is the digiscoper's friend but this isn't always the case,in fact bright but cloudy is far better as light areas of the subject get burned far too easily in sunshine, no matter what you try to do. Early morning and evening sunshine is good but you can often forget about quality results during the middle of a sunny day. Also remember that it's generally best to have the sun behind you (so in front of the bird) as you take the photo, though slightly off to one side is acceptable. More creative/artisitc shots are possible with the sun coming from behind the subject but it's not really feasable with the basic digiscoping cameras....and watch that the sun doesn't come straight down your scope, otherwise it will be a fried camera for sure.