Be aware that when the pot and soil heat up, the fine feeder roots actually cook, setting your tree back. To help prevent this from happening, avoid decorative pebbles and stones on the surface of the soil over the summer months.
It is more beneficial to put a moisture conserving material, i.e. coco peat or composted pine bark, pine needles or similar on top of your pot. Additionally, place two layers of carpet or similar under your pots, when thoroughly wet, the carpet will provide a degree of humidity betwen waterings. Note, extend the carpet 100mm beyond the pot you are placing carpet beneath.
Small Bonsai can be placed on a tray of water filled with gravel that will act as a humidity providing source between waterings.
Avoid standing your pots on concrete, brick pavers, slabs at all costs. This acts similarly to placing your bonsai onto a hot stove.
It is a good idea to relocate your more sensitive trees, i.e. Maples, Azaleas etc. to a protected position away frm the wind and afternoon sun.
It cannot be stressed enough that on hot days, bonsai must be watered at least twice a day. At least every second day, make sure you 'Deep Water". As the Japanese say: "First for the pot, second for the soil, third for the tree".
During Summer, organic fertilising cakes will provide all the needs for your trees to maintain good health and growth. Avoid high nitrogen fertilisers during this time, as the harsh sun can easily burn any new tender shoots.