Protect yourself with a high factor sunscreen and cover up with shirts and hats. At high altitude, there is less atmosphere to filter out the suns' harmful rays, so make sure you use a high factor even if it's cool. In hot climates you will also sweat more, so drink more non-alcholic fluids than you normally would, even if you don't feel thirsty and frequently re-apply sunscreen and insect repellents that may have 'sweated off'.
Always buy the highest factor sun screen that you can find. Then just use less of it than you would normally with a lower factor. This will make your bottle go further so saving you valuable space in your bag.