Scratch

Firstly you should make a variable called "Player Speed"

Now I presume that you are using one of the following blocks to move your player around:

<move(  )steps>
<change x by( )>
<change y by( )>

Well inside those blocks, put the Player Speed variable inside, and set Player Speed to how many steps you want the character to move.

Then when a player goes over a power up, you increase or decrease the Player Speed.

So it should now look like this:

<move(<{ Player Speed }> steps>
<set{ Player Speed }to( 10 )

<forever>
<if><touching[ Power up ]
<change{ Player Speed }by( 5 )

Hope that makes sense