I used to think that being 'hot' was that you had to go round preaching Jesus all day, be all the time in prayer and be a big mover and shaker in prophecy, gifts etc.
I think to be hot means to be devoted to Jesus, like you are to your husband/wife when you first meet them. Waiting for phone calls, letters, hanging on to thier every word. Its not about an 'outward show' but an inward show.
A woman whose fiance went away would not want to be seen hanging out or doing stuff she knew her future husband would be not pleased with, as it would show she did not care. I think this is the same with us and Jesus, we need to be acting like we love him, often Grace is used to cover disobedience, grace is not the power to cover disobedience but to overcome it.
If you are 'cold' you have a chance of re-finding your first love and getting 'hot'. If you are 'lukewarm' you are indifferent and apathetic. Jesus would rather us go totally cold than lukewarm.
Not teaching just thoughts.