how can i make my yoyo spin longer and spin smoother ;D
You could trade it ;).
Usually, it’s your throw, which will get better with time. Also, practice keeping the string from touching the inner walls of the yoyo while you are doing tricks.
A dirty, gummed up, or over lubed bearing can slow you down a bit.
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practice and to spin longer you practice it not the yoyo that’s the problem i throw hard your yoyo equals longer spin time! 
Throw better.
Seriously.
However, in some cases, there could be other factors involved. A good clean bearing goes a long ways towards enabling the yoyo to spin longer and smoother.
A string centered on a good, strong, straight throw is the best. Having a shaped bearing CAN help, but if your throw is not good, then you just have string centered on a bad throw, which is just as not good as with a flat bearing on a bad throw.
Practice. You WILL improve over time. The time is well worth it.