1. The Carrot Dangle. If you have a client that sends you small contracts with the promise of larger contracts- beware! If you have a potential client who asks you on two, or more occasions for detailed proposals without a commitment or counteroffer- the potential client is picking your brain and hiring a competitor. Clients who are not greatful of your services, promise larger contracts if you take lesser amounts of money now, or those who constantly need bids without every accepting an offer shouldn't be your clients. Don't get caught up in chasing the carrot. Go where there is real money and move on.
2. Be willing to say no. In fact, learn to say no. If you can't say no when a client is being unreasonable, then you're being bullied by the client. You don't want to be bullied. Dump the client and move on.
3. Clients who don't pay aren't clients. If you have a client who is on a payment schedule and this client is always late or behind- dump him or her and seek legal action. Clients who can't keep up with their payments are not your clients-in fact, they're problems. You shouldn't be working for free. It is a business, remember! So, if you have a client who is always late with a payment or behind - say see you later and find a client who is financially responsible!
Being in business is hard. And, it sucks when you have to let go of potential money because clients are leading you on, not paying on time, or becoming too demanding. In the long run, ditching these clients now will save you a huge headache in the long run.