1st thing i do is think about how i want it to look (shape, width, colour)
then i draw a concept out on paper, then eventually moving into photoshop starting from the background design and working up to were content is positioned (might sound weird)
always use shape tool as its easier to edit for future. once im happy with design chop it up into as less pieces you can (for eg. you have a nav bar thats a green gradient DONT use the whole image as its a repeated image so just chop it about 2 or 3 pxl and u can repeat the img. also when using images on your site only use JPG for high res images (banner, content image) and for the rest (logo, nav, footer etc. use png.)