Keep in mind that transparency and clipping paths are not the absolute solutions to every problem, and that Photoshop and digital imaging offer different techniques for different effects and results. Use transparency when it helps you complete a project rather than when it is interesting or fun to do. It isn't necessary all the time, and there are often alternatives to transparency use if it creates more problems than it solves.
Understanding Web color use, the properties of JPEG and GIF files, and image implementation via HTML is essential to using transparency effectively on the Web. There are some tricks to working with the edges of transparent floating objects. Become familiar with the defringe and matting commands.
The successful use of clipping paths really requires a good working knowledge of paths and the Pen tool. You need to adjust paths for fine-tuning, and may need to alter images somewhat to smooth and remove transitions.
Working with selection is also essential for getting the results you want. The method described here is just one of many that you might use. Expertise in creating and manipulating selections will allow you to more easily and accurately create the shape you want for your image.
Be aware of the additional image space around your transparent or clipped image. Though you might only see the area you intend in the final application, the file still retains all the pixels in the clipped or transparent areaand they take up memory. For better efficiency with the use of the images, crop the visible portion of the image as tightly as possible to the area you will actually be using. This will save load time in a browser, print-processing time, and file size.