Let me keep this concise for you, and tell you in the first line the secret to valuable LinkedIn endorsements.
Like everything in life the answer is in simplicity.
In asking every time you set up a personal skill, or give an endorsement -
"So what" - "What Does This Really Mean To Them, Their Colleagues and Potential Employers?"
and making that the endorsement you use or give, you will maximise impact for yourself and for the person receiving the endorsement.
Take for example endorsements like "ERP, or "Excel" - reality is this is nothing more than a superficial gesture unless it is in the right context. If we looked at a more effective endorsement we would come up with two answers:
Think carefully about the 'So What' and you'll come to the skill that is really the value add - I know if I were being scouted for a job I would much rather endorse or be endorsed for Business Process Improvement rather than only 'Excel' (which isn't to say the tool isn't important alongside it - read below. ..
Another key secret to Endorsements is thinking about the career lifecycle of those you are endorsing.
This is also why its mostly pointless to endorse someone you don't work closely with!
By considering the career lifecyle you can work out what the best "So What" is for that person. Let's take a look -
Notice that in junior, mid management and technical roles - the technical skills are exceptionally important - and may be the "So What" that employers and the receiver want.
Also note that people skills in most studies remain consistently as important whether you are in a junior, mid management or senior role.
I'm not sure if you've ever put any thought to the type of skills and endorsements you are giving, or being given - but there are definitely very specific types, all of which are in their own range of effectiveness.
A fun way to do it is the play the Endorsement Game, by assessing the criteria that are most important to your role and progression. The below is no science, just a guide to get you thinking:
Stage Setting - The 'what' the person does, and the industry.
Account Management, Sales, Social, SaaS, EMEA, ERP, Marketing...
The lowest value endorsement but still a necessity
Nice, Charming... Dreamy...
How you act
Prompt, Efficient, Effective
Technical Skills - Both Tools and Actions
Excel, Analysis, SQL, Microsoft Office, Reporting
Buzz words also fit in around this area - Big data, Cloud etc
Leadership, Management (Debatable...), Change Management, Team Based
Strategy, Strategic Thinking, Solution Selling
Outcome Based Endorsements
The ultimate compliment - when you or one around you has thought 'So What' enough times to find its impact on yourself, business or society
Business Improvement, Goal Achievement, Process Improvement, Outcome Focus