"Multitasking" is often seen as a desirable skill - I'm crap at it, btw, I have to turn the radio down when I go around a roundabout - yet multitasking is a very bad idea. Why? Because it makes each and every task take longer to complete, than necessary.
One of the most obvious (but, by no means the worst) causes of delay is the time lost when switching between tasks.
I love the following example, from Jon Cook, on the Critical Chain yahoo list, because it is so simple and concrete. It will take longer to read the instructions than it will to execute them.
Take a blank piece of paper and draw 3 columns.
You are going to execute 3 projects, first using multitasking, then not.
The three 3 projects are (1) write the letters A-J in the first column (2) write the numbers 1-10 in the next column (3) write the Roman Numerals I-X in the third column.
The end result is three columns filled like this:
A | 1 | i
B | 2 | ii
Start with the multitasking scenario - write in three columns, working row-by-row, across the page, starting with A then 1 then I, i the the first row, working down the page until all three projects are complete.
Now start the non-Multitasking scenario - write down the page A-J in the first column, then 1 - 10 in the next, and finally, I - X in the third column.
Try it and you see - it only takes 1 or 2 minutes - and observe how you save time by not multitasking. You won't need to time the scenarios - you'll just feel the difference.