Is there a recommended time to pause before transmitting, after changing to a new frequency?
I was always taught:


Change frequency 
Give it a good 3 count 
If no one is talking, transmit 


The FAA just advises a pause as per the AIM [4-2-1] but gives no time specifically


  a. Listen before you transmit. Many times you can get the information
  you want through ATIS or by monitoring the frequency. Except for a few
  situations where some frequency overlap occurs, if you hear someone
  else talking, the keying of your transmitter will be futile and you
  will probably jam their receivers causing them to repeat their call.
  If you have just changed frequencies, pause, listen, and make sure the
  frequency is clear.


You will also learn to anticipate the flow. For example if you hear a controller issue something you should expect to hear the aircraft read it back. Once you get a feel for the cadence of chatter you will have even less issues.