1. Use Google to Search Certain Sites
Google almost always does a better job searching a certain site than that site's own search engine does. If you are looking for cheap clothes from a certain brand or retailer and it isn't advertised much you can use Google advanced search operators to pinpoint exactly what you are looking for.
For example, if you are looking for discounted lonehoods from Styleuno you can try searching something like this: "discount" site:styleuno.com that is telling Google to search only styleuno.com for the specific word "discount". If you are looking for cheap hoodies on a site like Karmaloop with thousands of hoodies you can try something like "sale" "mens hoodies" site:karmaloop.com that is telling Google to search Karmaloop.com for a page that has both the phrase "mens hoodies" and "sale". The better you know the site the better you will be at this. For instance some may say clearance or 50% off etc.. but within a couple tries you will find something.
If you don't have a specific site or brand in mind and you just want some cheap clothing you can use a different advanced operator to search online. For example if you want $20 hoodies you can try this: intext:"$20 hoodies" and that will pull up any webpage that has the exact phrase "$20 hoodies" written in the text of their website.
One thing to take note of is that there is never a space between the colon and the first search phrase!
2. Google Alerts
One way to be notified about any products going on sale is to create Google Alerts. If you don' know what a Google Alert is just go to Google and search it. It allows you to enter a search phrase and you will be alerted by email anytime that search phrase appears on a website or blog or in the news. It also allows you to customize this as to how frequently you want to be notified. Always click "As it happens" if you want to be notified as soon as something goes on sale.
For instance, if you're waiting for a sale on Forever 21 clothing your alert will look something like this: