(Metal fixed axle yoyos do not need any maintenance other than to keep them clean and change your strings regularly)
The length of your string can have a huge impact on how tricks are performed. A short string results in quicker, more gentler loops, whereas a longer string results in slower, more powerful loops. Generally, a shorter string is considered to be easier to loop with than a longer string. Longer strings are normally preferred for string tricks because they give you more room to work with. However, ultimately it depends on your own style of playing and preference to whichever string length you use. For beginners it is recommended that you have your string no longer or shorter than from the ground to approximately 2" above your waist.
Strings should be changed regularly and yoyos do play a lot better with a new, broken in string. A good way of breaking in a new string is to firstly wrao it a couple of times around the metal axle of a yoyo, then pull it through. This has the same result as if you were to play with the string on the yoyo for a couple of minutes.
Strings also come in different thicknesses; from Slick 6 to type 10 or even higher. Slick 6 and Type 8 strings are the two most commonly used types of string. Slick 6 is made by Dale Olivers' company Spintastics. It is a thin type of string which does not need to be broken in to play well with. Because of it's thinness, it is excellent for string tricks immediately. It is not however, good for looping, as the loops tend to go higher and higher as the string wears.
Buy your strings in bulk if you can. They can come in packs of 100 and can save you alot of money in the long run. You'll also never be short of a string when you need one.
Try out as many types of string that you can to help you decide which type suits you best. There are many brands of string on the market, but you'll have to work your way through them before you know which are your favourites.
Type 8 string is probably used by more people than any other type of string. It is brilliant for looping with, and is good for string tricks once it has thinned a little. You'll probably end up with a 'knuckle buster' if you use a brand new type 8 for string tricks with the majority of yoyos.
Vaseline or Vaseline Lip Therapy is a very good substance to use on both nylon and ball-bearing yoyos. It is not overally powerful but it does increase responsiveness a great deal. The downside to vaseline is that it doesn't last for very long and will get your fingers messy. You may find you have to re-apply vaseline every 30mins to 1hr. It is however, available just about everywhere and isn't too expensive. Wipe your fingers after using vaseline so that you avoid getting any on the yoyo halves.
Brain Lube is a thin substance that is teflon based. It is thinner than vaseline, so you don't get quite so much slow down on your yoyo. It comes in a convenient tube pen, but is still quite messy and is likely to get on more places than just the axle or bearing, so keep a tissue handy! Useful on nylon bearing yoyos but great for ball-bearing yoyos. It also helps to keep the bearing quieter in operation. Fairly expensive at £5-6 but you do get 5 Yomega strings with it if you buy the maintenance kit, and it will last you a very long time.
Dragon's Fire is a red grease which is made in Wales. It is used by James Appleby, UK Champion 1998, and is probably the best lubricant you can use next to Super Lube. It comes in a syringe with a label on it stating its name and the fact that is Professional YoYo Grease. It really does work excellently with ball-bearing yoyos, and is not too bad with nylon bearings. Again, like Super Lube, do not apply too much of this stuff to your bearing; a couple of drops is more than enough. Dragon's Fire is what I most regularly use. Unfortunately, it is only available from Martin Ashfield Cycles in Wales at the moment, and I'm not sure if they ship overseas, but if you can get hold of some, then don't pass up the opportunity to do so. The total cost of one syringe and shipping in the UK is £3.50. This is an excellent price, well worth it for your yoyo. The syringe isn't very big but it will last a fair while.