If you have access to email on your phone, you might find some websites hard to use due to the layout of the site. If links are too close together, it can be hard to select the correct one first time. The same applies to emails too and I picked Expansys out as an example because I think their menu could do with improving.

In order to get maximum usability you should ensure that each link has "breathing space" - that is, plenty of non-linked area around it so that when you select it, you'll always select the correct link. That might mean giving your menu a bit more vertical height (usually made skinnier to maximise the above the fold area), or increasing the menu's text size (both the former and latter depend on whether you've used images for your menu). You could also try reducing the amount of menu items to ensure a good amount of vertical space. On emails there's usually between five and seven menu items, but to ensure usability on phones, you could try using a maximum of five.

This Expansys menu is a great example of one that could be improved - increasing the font size by 2px and removing 2 menu items would make it a lot more usable for people with smart phones - kinda funny that they haven't thought of this considering they sell phones.