Random collection of stuff Ive figured out in the last 7 years in the personalisation and decorating business. I hope it helps, its in no particular order and is mainly aimed at people who want to sell to earn money
Design; Is the key to success, good designs sell much better than poor ones. Learn about design, make sure you understand what is and isnt good design. Learn to use good graphics software proficiently. A good design on a poor quality substrate will sell better than a poor design on a good quality substrate. A poor design on a good quality substrate is a waste of the substrate.
Price isnt the only reason something will sell (see design info above) its an important part though, unless you can differentiate your product from your competitors 9 times out of 10 they will buy what they perceive to be better value. Customers want to buy the best quality its only price which stops them, the more the customer wants something the more they are prepared to pay.
Buy Good quality equipment, if you cant afford to buy good quality then buy second hand branded or save up till you can, making do with inferior kit is a false economy. A press at £1500 is dearer than an inferior press for a reason (same size features) it will produce better quality results more consistently for longer. Equipment is also more expensive the more productive it is and the more people it replaces. Having the equipment doesnt guarantee success, however you will not be as successful as you could be if you had better quality kit.
Specialise; It is very easy to get distracted. Do one thing well, if you're a mug person make great mugs, if you want to expand make more mugs rather than add a cap press for example. When you are making and selling so many mugs and generating more cash than the mug decoration needs, then expand with a complimentary line and equipment. Having more lines sounds like a great idea however it spreads your money and your focus really thin. Find a niche and be the leader in that space
The way to reduce costs isnt necessarily buying cheaper inks paper blanks etc. The best unit savings come when you can produce more product per hour of production time with the minimum amount of labour. Making 70 mugs per hour with one person is much more efficient than making 50 mugs hour with 2 people and less efficient production methods/machinery. Big is better, wide format presses/ printers are more economical than their smaller counterparts however you do need the work to put through them. Sales trumps virtually everything, provided you have sufficient margins to make the rest of it worthwhile, look at the return on time its no good netting £200 a week if it takes 100 hours of your time to do it. Know your numbers, profit margin percentages mean absolutely nothing unless they are attached to a number.
Hope some of this stuff may be helpful.
Good luck