Hi there,
I have experience of being in Israel in July and must admit I found it unpleasantly hot. However, I'm not a fan of the heat and burn easily because of being pale-skinned so my idea of "hot" is probably different to that of people who CAN handle the heat! I was there from around 10 July onwards and found it unbearable, although it WAS slightly cooler than Jordan (where I was immediately before entering Israel). Jordan was 35 degrees celcius so I would guess that Israel and the West Bank was 30 or 32, but on the day I visited Jericho I was warned in advance that the temperature would be 40 degrees and I believe it was. I found Jericho hotter than Jordan but, thankfully, many of the more touristy places have AC.
As for advice, I'd suggest that you carry a large supply of water with you.
NEVER head into desert areas (or even Jericho) without at least 1 bottle of water.
Never head into the desert without a guide. It's very easy to get lost and disorientated in remote desert areas and it could be a long time before you are found!
Wear a wide-rimmed hat or some other light-coloured head covering, try to avoid wearing nylon - cotton clothes are more practical and comfortable.
Sunglasses help, the sun gets extremely bright.
If you burn easily (like me!) then be sure to apply sun cream before heading out for the day.
Try to avoid very heavy or fatty meals. I found that they just made me feel more sickly in the heat.
Instead, I suggest eating lots of fresh fruits.
If the heat bothers you very much, try to avoid going out in the afternoon. Mornings and evenings are slightly cooler.
If I think of other advice I'll post it on here! Hope you have a good trip. :)