What are the consequences of fouled spark plugs in flight, and how can I clean them?
You should read up here but generally speaking plugs are less likely to foul in flight (and is why its not talked about much). Plugs often foul due to the mix of low engine temps and high led content during warm up at full rich settings. This is why a leaned out warm up is advised for some planes, you should talk with your instructor about the plane you are flying and what the proper warm up procedures are. 

Please consult your POH for any airframe specific emergency checklists that may apply to your aircraft. Generally speaking if you depart with a fouled plug and it is causing a rough running issue you should cycle the magnetos untill you are running on the good set of plugs and get the plane down safely. This is most likely the result of a poor or non-existant run up check. Unless you are sitting in line for a long time after your runup, you should generally be doing runup close to departure time.