If you want your puzzle to be really hard here are some things to try:
Avoid words that read left to right (for languages naturally read in that direction),
top to bottom (in all languages I think), and horizontally or vertically when at the edges of the grid. These are all easy to spot even if no clues are provided.
Giving a list of words to find makes things a bit too easy. A hint at some connection or theme if the words are really obscure might be okay. Of course if you want your puzzle to be really hard just use the most obscure words you can think of and only provide the number of words to be found (if you do this you will need to check thoroughly that no extra words have crept in.)
If you feel the need to provide a list of words use fragments of those words to fill in blank spaces. Placing the start fragment of a word near the beginning or end of a complete word is great way to create a wild goose chase. Fill the remaining single spaces with the least common letters from your clues, you want to make sure that if a letter only appears once or twice in all your you use it at least three or four times to fill in blank spaces. Any strings of letters that appear repeated across many of your clues will be any easy target to search for so make sure you use those to fill spaces. The more red herrings the better.
To make a puzzle really time consuming make it really large. Spotting a word and finding another that begins or ends nearby, or crosses it, makes things too easy. Having a grid large enough to space your words out nicely can completely eliminate this.
If capitalization is not important to any of the words, randomising between upper and lower case can make a word search fiendishly difficult to read. if you want a puzzle that is really difficult just to read, try a textured background and patterned letters. I'm thinking of camouflage schemes for the letters and background to are so similar you can hardly see the letters.
To be really unfair why not swap some letters for numbers or symbols, 0 for o, ( for c, that sort of thing.
A most wicked twist is to allow words to wrap around the edges of the grid. This is particularly unfair if there is no reason to expect you would do this. So to be really cruel make sure you give no hints this is what you are up to.
Of course if you are not a complete sadist then ignore this advice. If you want to make a nice easy puzzle do the opposite. A good word search will have just the right amount of cruel and kind elements. The right balance depends on your target audience.