Sometimes worship just clicks. Sometimes you get into the service and the songs just seem right, and you feel how in love you are with God, and your voice seems to sing all the right notes, and you don't get distracted, and tears are streaming down your face and your heart is full and it's just amazing. Those are of course everybody's favorite kind of worship time, and I believe we should work hard to learn to do that every time if possible. Does that mean there's something wrong in times when it's too "easy" or too "hard" and you don't reach that place? Well, maybe, or maybe not. Maybe it's just that sometimes the Holy Spirit chooses to move in certain ways, and sometimes He chooses to move in other ways.
Actually, maybe the real deal is that we need to be ready to worship God in whatever way it comes. Maybe it's part of "being ready in season and out of season." One of the main things about sacrifices in the Old Testament was that they be perfect, pure. Lambs had to be spotless, no bones broken, nothing physically wrong with them at all. If praise and worship are to be one of our "sacrifices" that we offer to God, I would say that the main qualification is that the sacrifice is pure. Offer your worship with a pure heart, pure motives. I'm sure sacrificial lambs often got dusty on the walk to the Temple; sometimes we pick up crud in our daily lives too, and sometimes it's tough to shake. But God can see what's in your heart, and if your heart motive is pure, your sacrifice is pure. Even if you get the words wrong and sing the wrong notes. Even if you find yourself thinking about lunch buffets halfway through the last song. Even if you yawn during "Open The Eyes Of My Heart." Approach God with a pure heart, and when you get to the top of the mountain, He will be there to meet you!