New guitar students often come to a lesson and say something like, "It should be in tune - I tuned it yesterday." I suggest that this is faulty thinking, and that you will be well served by checking your tuning every time you pick up your instrument. I tune my guitar several times a day.
Two of the most consistent failings of guitarists are their tendencies to play out of time and out of tune. To a certain degree, the latter is due to the instrument - every guitar tends to be in tune with one chord and slightly out with all others. However, many players simply don't pay attention to whether they are actually in tune.
Playing in tune is fundamental to playing well. Every time you pick up your instrument it should be the first consideration.
Most learners find tuning difficult. As with playing music on any instrument, the skill lies in developing a 'good ear'. This can be learned and is a skill which develops the more you play. You'll find detailed instructions on how to tune by ear in this post.
Half the battle lies in having a good attitude. You want to be relaxed and approach it calmly - don't hurry the process or you're likely to get it wrong.
Always tune up to the required pitch, never down. If needs be, tune slightly lower than required and then fine-tune up. If the string is ever so slightly sharp, try gently tugging it to lower the pitch slightly.
Most guitarists who consistently suffer from 'slipping strings' do so due to incorrect fitting of the string in the first place. When fitting strings - and prior to tuning - it's useful to 'stretch' each string, pulling it away from the neck and 'tugging' it a few times.
The lighter the strings, the easier they will lose there tuning. Other circumstances which result in de-tuning include knocking or dropping of the instrument, change of temperature and constant 'bending' - particularly with new strings.
The best approach is to loosen each string, get them all roughly in tune and then fine tune them. You should make a point of remembering which note each string is tuned to and obtain a reference to ensure you are in concert pitch ie a tuning fork or pitch-pipes.
Don't rely on an electronic tuner all the time. The art of playing any instrument well depends on training your ears to hear properly. Learning to tune is an excellent method of training your ears which I encourage you not be avoid.
In my experience, a guitar that is regularly and frequently tuned will be easier to tune and will 'hold' it's tuning for longer.