There's really not a whole lot you can do to keep the image quality once you size down an image. The smaller you scale it, the more the program has to squish the pixels together.
Some things I can suggest are:
1) Adjust your resolution when you first open your canvas. For example, when you create a new canvas in photoshop, the default resolution is 72 pixels/inch. I usually up this to 200-300. There's no right or wrong amount, but the smaller the number, the lower the resolution, the higher the number, the bigger the file size. You should try to meet yourself in the middle. I'm sure every program has a similar feature.
2) Don't work on an extremely large canvas for an image you know you're going to be scaling down to DP size later. Working on bigger canvases can help with the smaller details, but if you're working on a canvas that's 1920x1080 and you scale it down to 160x220 with respective cropping, 60% of your details are going to disappear or be hardly noticeable.
3) Avoid uploading your finished images on websites that automatically convert your image to a .jpg. (Like tinypic and imageshack.) Saving as a .png will help keep as much resolution as possible considering you already sizing down the image, but that quality will be ruined if you don't pay attention to your image host. Converting to .jpg will blur the images. The amount it does depends on how much compression the image host applies.
Left - Photobucket | Right - Tinypic