1) Monitor your advertising - ask new customers where they heard about you.
2) Repeat adverts do work, Repeat adverts do work.... Research has shown that you get better responses from a series of adverts, often the 3rd or 4th time a potential customer sees your ad is the time they respond. So if you have a fixed budget, go for a run of smaller adverts rather than 1 or 2 large ones.
3) Grab them with the headline! Try and have a heading or prominent key phrase near the top of your advert to make it clear at a glance what you are offering
4) Appropriate pictures and images help. The human brain is very good (and quick) at recognising images where as time is required to absorb a lot of text.
5) Don't write a book. It's easy to get carried away and write lengthy prose about your products and services but readers can get bored and won't always bother to read a lengthy description. Try using bullet points, refer to your website or just give enough informations so that potential customers will call you, then you can give all the detail.
6) Landlines are good. If possible, don't just have a mobile number in your advert. Having a landline gives an impression of an established and reputable business.
7) Remember adverts can be to gain sales directly but also for "brand awareness" - getting your business known so that when customers do need your product or service, your brand is the first they think of.
8) Update or refresh your adverts periodically.
9) Remember who your target audience is - design the advert and choose where to publish it based on that. Put yourself in your customers shoes.
10) Persevere, don't be put off if you run 2 or 3 ads and get little response (see points 2 and 7) and consider timing or seasonal campaigns.