You are the face of your company & need to be polite/friendly at all times. Today however you feel tired/grumpy. How do you still give great service?

1. Stop for coffee on the way to work.

2. listen to a favorite piece of music while taking route to the office

3. Socializing for5 minutes at the start of your workday before requiring to knuckle down. 

Put a smile and avoid any unnecessary discussions . And make something to take myself out of this mode like drinking cup of coffee hearing a song  i like , order fast food in the break and eating with my colleagues .

Rule that I set for myself whenever I go to work is be an actress,portray somebody else and leave your BS to your house even if your whole world is torn apart. It will only take a second to ruin your career and yourself when you act like a BS so better to forget you are tired and grumpy and do your job and if you cant do it and handle stress and pressure then quit and find the best job that suits you.

If you feel tired or grumpy - then a nice cup of tea or coffee should start the day - a few tricks which also help is imagine you eating your favourite dish or watching a cartoon like Tom and Gerry - the smile automatically comes to the face - try it ..... you will not be disappointed.

first of all I believe that your feelings must be reflected to your relation with the customer

I believe also that feelings Can not synthesize 

my advice , if you are in bad mode , go away little bit , do something makes you happy , stop thinking any matter bather you , and continue  :_)

With smile deals polite & friendly to the client any time,  dont show your tiredness to the client.

Take a break sip a coffee these days or if feel so tired i would go for an energy drink. Relax and inhale fresh air.. I am sure you will feel great and energetic to give a estemeed service.

You leave your grumpiness at the door and put on a smile. If it gets too much during the day, take a short break to compose yourself.

Keep that smile as usual so that no-one notices the change.

Personally when i feel tired I usually try to satisfy customer needs cause talking to customers make be feel great cause communication with your customers is a positive side that reflects on you and let you provide good service.Sometimes when talking to your favorite customer relax you and change your mood , contacting with your colleagues push you up and change your mood completely.