Sparring is very important in Muay Thai training. It is the best chance that you can try out the moves or techniques that you learn, improve the ones that you are not quite confident to use (in this case you should use those moves more often to gain confidence), know what to do and what you should not do in the match.
To have an effective sparring, all boxers must control themselves. This means you will have to watch out for your partner because it is training still, not a real fight. You want to be able to learn from sparring not defeating your friend.
If you know that your partner has less strength than you, or is lighter or slower or has less experience in fighting, control your power and use light contact, even better if you note which techniques or moves that are too dangerous to use to that partner.
More importantly, even if your partner has the same level of skills, you must control your temper and fight carefully. Control your power if you decide to spar in rounds to be able to stay until the last round. Don’t get wasted and over use your power at the first round by sparring too intensely when not necessary.
Learn and get the best from sparring sessions by watching other competitors and see what techniques you can use in real competition.