Sometimes we use Photoshop, other times MS Visio. Is there any tool to design the User Inteface that is intuitive and easy to use? Also, is there any best practices.
What we want to accomplish is a simple and easy to see image of how the actual UI should look. We don't want to deal with any programming at this point, since this is done even before the estimate is done.
Depends on how far you want to go with the design. I actually find that Balsamiq is the best tool for doing estimates and keeping the customer focused on what really matters. It's extremely easy to get complete mockups in mere minutes.
You should treat the true design the same as you treat the development - colors, images, fonts, details, embellishments are all implementation details and time-consuming work, and are exactly what tools like Fireworks and Photoshop are designed for - high-end, detailed work that require professional applications to accomplish. Just like development.
You can also look at the 960 grid system to help the design process.
If you're just laying out the controls (that is, not dealing with graphic design issues like typography, colors, etc.), I recommend the the trusty paper+pencil combo. It shows people that it's not the final design, as opposed to badly spaced and randomly colored, real-looking controls that may look like a bad final design. It's also easy to draw and easy to discard - an advantage for any initial design.
If you can't draw straight lines well, then you can use dot graph paper. If you google for "dot graph paper" you'll find many sites that can produce dot graph paper PDFs you can print (I recommend making the dots as light and small as possible, btw).
If you already have the layout, and just want to make a realistic-looking mockup of the final product, then Fireworks would probably be my first choice. They have a nice GUI-controls symbol library, and bitmap/vector editing features that are similiar to Photoshop's and probably much better than Visio's (although I haven't used it much, so feel free to correct me). It also has some nice unique features like 9-slice-scaling (to draw round corners) and much better slicing tools than Photoshop's.
I've used Axure. It's costy but you can use it to do stuff quickly and easily : http://www.axure.com/