Try not to have any conditions to it - there was a section on my parents' LPAs which said things that caused us problems.
* don't have words like only to be used if found mentally incapable (this would need a doctor to confirm and they would charge you for it. It is also not necessary as you can use it as soon as it is registered because your family member agrees)
* if you are not the sole attorney make sure you can act Joint and Severally for everything. Again we had problems because our wording was something like "joint and severally except in the case of property/assets, where attorneys are to act jointly"
M&S bank took property/assets to include stocks and shares so we had to all send ID proof.
You are aware of the fact that there is no requirement for a solicitor? If you feel better having one and don't mind shelling out hundreds of pounds, fair enough, but you can fill in these forms yourself, there are plenty of guidance notes and charities to help you - it's not rocket science.
Keep it broad because at this point you're unlikely to be able to think of every eventuality. I am regretting some of the language in my mother's because it talks, albeit in vague terms about being unable to look after herself and it's becoming increasingly apparent that mine, my family's and friends' ideas about 'looking after oneself' are clearly at odds with both social services and the memory clinic who believe not washing, not changing clothes, not eating and urinating in a washing up bowl are perfectly acceptable behaviours.