The simplest and most important part of yo-yo maintenance.  In normal use, string will get dirty, frayed and worn surprisingly quickly, particularly so if you're doing string tricks.  This situation is only exasperated when the yo-yo is used outdoors or in a humid atmosphere.  Once a string begins to get worn, it will then start to perform eratically, making tricks harder to accomplish.  A worn string is also more likely to break - most probably sending the yo-yo on a path to destruction in the process.  Unfortunately, the only solution to this is to replace the worn string with a clean new one.  Make sure you always have lots of spare string!

When new from the factory, bearings are lubricated to maintain performance and to prevent rusting.  The exact amount of lubricant (lube) shipped with the yo-yo will dictate its response level and spin time, which may or may not be to your liking.  Also, after extended play, your bearing will inevitably get clogged up with additional dirt and grime, which will ultimately degrade its performance.  To deal with these issues, you'll therefore need to periodically clean the bearing.  To do this, first remove the bearing from the yo-yo (see below if this is a problem), then bath it in lighter fuel.  How long you leave it in the fuel will depend on how dirty the bearing is and how clean you want to get it.  When you've waited long enough, remove the bearing from the liquid, dry it thoroughly with a paper towel, then manually spin it on the tip of a pencil for several minutes to get it running smoothly again.

Some players like to remove the protective side caps from a bearing whilst cleaning it.  Although this does allow you to get the bearing even cleaner, it's a matter of opinion as to whether this is really worth the effort.  Be aware that, by removing the caps, you'll stand a chance of damaging the bearing.

Whenever a group of yo-yo players get together to discuss their art, you can pretty much guarantee that the subject of 'response' will arise soon after.  The responsiveness of a yo-yo is a measure of how easily it will return to the hand from sleeping.  Although it's natural for beginners to assume 'responsive=better', the reality is more complex and very much a matter of individual taste and style.  Some players prefer a completely unresponsive yo-yo, others don't.

To help control yo-yo response, manufactures include a variety of 'response systems' within their products.  These include Starburst (plastic ridges molded into the shell), 'O Rings' (rubber rings recessed into the shell), a variety of sticker-type solutions (as shown) and even mechanical devices, such as that seen on the FAST201.  Whilst 'O Rings' and Starbursts offer little scope for adjustment, different types of sticker can be employed by the player according to their style.  As such, if you're the sort of person that likes to tinker with a yo-yo's response, a sticker based yo-yo (such as those from Duncan or Dif-e-yo) is probably going to be your preferred choice.  Be aware, though, that some stickers don't last too long, so you'll have to budget for replacements.  Another more recent innovation is that of the 'hybrid' response, which involves having a starburst on one shell and an 'O Ring' on the other.  Don't forget that you can also use the cleaning and lubing techniques mentioned above to alter a yo-yo's response.

The process of removing a yo-yo bearing usually just involves un-screwing the two shells and letting the bearing slide off of the axle.  However, there are some types of yo-yo (principally from YoyoJam) where the bearing is clipped into a recessed hole in one or other of the shells.  Whilst this design is good for ensuring that you don't accidently lose the bearing, it does make removing it for cleaning rather more difficult.  Probably the best and safest removal method is to apply leverage to the middle ring of the bearing.  You accomplish this by inserting a tube into the centre ring then gradually rocking it until it becomes free (see image).  The only problem with this is in finding an appropriate bit of tube (a custom tool is available from yo-yo suppliers).  Alternatively, you can use a socket set to remove the bearing by applying leverage to the outside of the ring.  This works well, but may cause damage if you're too being heavy handed.  A gentle rocking motion is all that's required, so don't over-do it!  Should you have a concave-shaped bearing, you might find that the socket method doesn't work.  In this case, cover the bearing with a thick cloth (so as to protect it) then use needle-point pliers to ease it from its hole.  Finally, I've heard talk of a 'string method' for removing the bearings.  From what I can gather, this last method is best avoided.