Keeping your bonsai watered while you are away from home has always been problem for enthusiasts. Pruning, feeding and general maintenance work can be left for a week or two without any detriment to your trees but watering has to be attended to on a daily basis. I have known a few enthusiasts who have refused to go on holiday during the growing season for fear of losing trees through lack of water! Fears of tree's drying out in their owners' absence seem well founded as one always hears stories of well-meaning friends, relatives and neighbours who forget to water an expensive, prized tree......

I have yet to hear of a reliable way of watering indoor trees. Asking a friend or neighbour can work but only if they are reliable enough to remember to water properly on a regular basis. Many trees have been lost after neighbours have forgotten to water a tree on just a single occasion while the owner is away on holiday. Trying to water trees using a wick is very haphazard and leaving a tree sat in water is likely to cause root damage, particularly for any trees that are already weak or in poor health.
If an indoor tree has to be kept inside during the period of your holiday (tropical species during the Winter for instance), the amount of water the tree requires daily can be reduced by watering well and then being moved to a cool position away from light to reduce transpiration and moisture loss. This will increase the amount of time the tree can be left without additional watering, hopefully for the duration of your vacation.
If a tree can be defoliated with confidence, do so a week before their last watering is due (moisture needs can increase greatly in the days immediately after defoliation).

It is far easier to holiday trees outside. Position all trees together out of the wind and direct afternoon sun (both of which can cause excess transpiration and moisture loss), collect all trees together (especially useful if a neighbour is doing the watering; having the trees in one area of your property avoids individual trees being missed by the neighbour).
Neighbours may be helpful and have the best intentions but are unlikely to have the experience to know how much and how often your trees require water and more importantly, how quickly the soil can dry over the course of a day, particularly when it is hot or windy.

I have found that a temporary automated watering system is the most reliable way to keep bonsai watered for extended periods. Garden sprinklers are positioned around the bonsai and connected to a mains tap; the tap is left open/on. A programmable water timer is placed in between the tap and the hose/sprinklers, this contains a valve that stops the flow of water from the open tap into the hose/sprinklers, until, at preset times, the water timer valve opens and the flow of water passes through the hose to the sprinklers, watering your bonsai.
Water Timers can be preset to allow watering 1 or more times a day for anything up to ½ hour at a time. Most Water Timers are powered by battery and so are very easy to install when needed; no connection to outside mains electricity is necessary.

Set the timer so that the trees will be slightly over watered and ensure that under watering will be completely avoided. Short periods of over watering will not damage a bonsai, any period of under watering (the soil drying out) may cause irreversible damage.
When presetting the frequency and duration of watering, don't rely on weather forecasts to try and prejudge the water needs of your trees. Always assume rainfall will be zero and temperatures above average for the time of year.
Few garden sprinkler(s) will give out a even spread of water; there will be areas that receive more water than others; try to use these for particularly thirsty trees.
Finally, always make sure a new battery is inserted at least once a year!

This method of automated watering is also labour saving but is not ordinarily recommended for watering your trees, over watering is a big issue over prolonged periods of time. However, as a short term solution, it is the most reliable way I have found of ensuring your bonsai will survive while you are away from home.
My system consists of a £30 water timer, a length of hose and one £5 garden sprinkler and has successfully watered over 200 bonsai and potentsai, 3-4 times a year for a number of years. As a result of using this system, I have yet to lose a tree while away from home, even during one particular two week period where temperatures had continually exceeded 80F and reached 100F on a couple of occasions