One little tip... If you're are either cautioning or sending a player off for his use of language, never say it back to the player.
If he/she asks what the card is for, simply refer to the incident but use the phrase "foul language" or similar. Don't repeat what the player has said.
The only time I repeat what a player has said to me is if I'm an assistant and the referee has asked me what happened. I do that quietly and only with the referee by my side.
Simply put "player, you have now produced fouls on three separate occasions. There *point*, there *point* and there *point*. Therefore I will now be cautioning your persistent infringement of the laws. Can I have your name please?" "Yeah you're right ref, I've been a bit silly. My name's Joe Bloggs. Keep up the good work ref, you're having a blinder".
Sometimes depending on my mood, I would call in the captains and tell them why the player is getting cautioned or sent off without even talking to the player in question.
Don't be rude, remain calm and talk to the player like a human. Explain your decision in a manner in which will not escalate the situation and be sure he/she understands the situation. Manage the situation and when the time comes for issuing a card, it will come naturally. At the end of the day, if it's due to a rubbish challenge, 9/10 it would have been made because the player lacks the technical ability (particularly at grassroots). It may have been accidental so don't go off on one. I always talk to people with a respectful, calm but stern tone.