Informing clients of a rate increase?

Do all of these, not just one or two.

• Inform/remind them about the benefits that you have been offering so far, and why these benefits are superior to your competitors. In other Words, re-sell to them.
• Tell them that external circumstances forced you to raise prices, otherwise you can't continue providing a good service. Specify the external circumstances; it can be market situation, rising costs, business expansion, etc.
• (This is especially important if your rate increase isn't caused by external circumstances). Provide them an additional value that doesn't raise your cost. This can be a bonus hour or service, paying more attention to their needs, improving your services (can be quickly done by upgrading your skill - e.g. read a book), etc.