I have done this process countless times now and am often asked by friends to help out (as Mavericks has caused a few errors to occur with older models) so I think it's about time I made a post that explains how it is done. Be aware that you shouldn't really have issues on the newer models and this is also a guide for installing from a USB and not on one!

Three things you need:
1. USB (>=4GB).
2. Preferred Windows 7/8 .ISO (I prefer 7)
3. 20-40 minutes.

Step 1 - Enabling USB Booting on Bootcamp

If your bootcamp already has the "Create a Windows 7 or later install disk" option you can skip this step, otherwise this is completely necessary.
Bootcamp Menu Image

Navigate your way to Applications>Utilities, right click on Bootcamp and click Show Package Contents. Now click on the Contents Folder and open Info.plist in the file editing software of your choice (I prefered TextWrangler but have since grown to love Sublime Text 2).
Navigating Menu Image

Now that this file is open you need to find this line of code:
<key>_PreUSBBootSupportedModels_</key> and remove the "Pre" so it is just " <key>_USBBootSupportedModels_</key>. We will be editing the code just under this so your model will be allowed to use a USB to install windows but first we need to find out the model. Model Indentifier

Click on the apple in the top left of your screen then About This Mac > More Info > System Report... The Model Indentifier is what we need here. Just copy and paste that in between two new string limiters on a new line in the Info.plist file. You can see mine is:

Go ahead and save this file when you've done the same. It will prompt you to enter your password so do that. Bootcamp seems to not open on some models after this so if it isn't for you go ahead and open terminal and copy and paste this in:
sudo codesign -fs - /Applications/Utilities/Boot\ Camp\ Assistant.app (press enter).

Step 2 - Creating Windows USB

This step is very simple as all you have to do is insert your USB, open bootcamp and have only the "Create a Windows 7 or later install disk" ticked. It will ask you to find the Windows.iso (may automatically have it selected) so don't forget about where it is. This should take no more than 3-4 minutes.

Step 3 - Installing Windows

I suggest downloading the windows support software from here http://support.apple.com/kb/dl1638 as downloading it from bootcamp can often freeze or fail. This support software contains all the drivers that make windows run smoothly on your Mac. I would also suggest throwing this on a separate USB so you can quickly install it after Windows is installed. The alternative is to download it on windows once it is installed.

Now go to bootcamp again and have just the "Install Windows 7" option ticked. The first thing that will happen is it will ask you to partition your HardDrive. Set the size to whatever you think you need but be aware that to change this after, you have to remove and install windows again (get it right the first time).

Once the partitioning is done your Mac should reboot and open up with the windows installer. For the most part you need to just follow the options here. The installer should ask you where you want to install Windows and this is where you need to simply click on the Partition you want to install it on, the click options and then format. Once this is done you should be able to click next and then the rest is all you.
Windows Install Step

If the install seems to freeze in one spot for a few minutes it may actually have frozen (happens me about 20% of the time). There are two ways to fix this:
1) Plug in a USB keyboard and push a button on it every few seconds.
2) Power off your laptop and open up your Mac partition (Hold down option on start up so you can choose). Open bootcamp Remove Windows and begin Step 3 again.

Step 4 - Installing Windows Support Software

This step is necessary because most of the support software bug out while they are installing. First of all you need to extract the Windows Support Software file you either downloaded before the install or have just downloaded now to your desktop.

Now you need to open up cmd line as an administrator. To do this click on the start menu and type "cmd" into the search bar. Now because you don't have drivers installed yet you cannot right click on cmd so to do this you must either plug in a mouse and right click using that or hover your mouse over it and press fn and f10. Now click run as administrator.

This is where the most basic of cmd line knowledge becomes useful. You need to change the directory to where you extracted the Support Software (Open the folder and put click and grab the Bootcamp folder to your desktop if it isn't already). To do this type the following while pressing enter after each block.

cd \Users ENTER
cd USERNAME ENTER (Your Username)
cd Desktop ENTER
cd Bootcamp ENTER(This was my file name)
cd Drivers ENTER
cd Apple ENTER
Now that we're here all we have to do is run the bootcamp.msi file.start Bootcamp.msiENTER.
cmd instructions

This should install all your drivers and after a quick reboot you should be done!

A very handy piece of software that I think makes life a lot easier from this point on is called rEFInd. rEFInd will appear anytime your laptop starts and give you the option to boot into any OS on your laptop. There is a lot of software like this but as this is the only one I have used, it is the only one I can recommend. http://www.rodsbooks.com/refind/

rEFInd Image