In no particular order
Good accountant who has knowledge of your field of work if it is specialist
Keep up to date with all paperwork
Declare everything
Offset all allowances of expenditure against accounts
Don't be afraid to ask for advice
Working alone can be lonely / sometimes can make you vulnerable
You need to be a workaholic (or at least to get you going )
Know your target market and sometimes read between the lines as to where the money comes from and why
Do not undersell yourself - I use several specialists that have eye watering fees but an hour or two of their knowledge / I put can earn me / save me many times their hourly fee so what seems expensive is only relative
Do not spend hour upon hour working for nothing because the client is a friend / family
Keep work professional and family / friendship separate as much as you can
Work out costings and fees very carefully- often the hidden costs are overlooked
Don't run before you can walk , build your business slowly and carefully
Talk to other self employed people in all types of enterprises, there is always something you can learn from them
Don't be afraid to change direction , diversify , look to the future and plan ahead but be open to change
Save when you are doing well as you never know what's around the corner
If clients meet you / visit your premises a smart but modest outlook ( personal turnout / office/ car etc ) will make you look successful .