Possible can give the impression of uncertainty and lack of knowledge. If you think something is possible contrast it with facts to sound more authoritative:

• It is possible to increase production but we need to assess the machine capabilities.
• It is possible to increase production but we need to assess the machine capabilities and this should take about 3 hours.

• It’s possible we can get the money for the expansion. [weak]
  
• It’s possible but before we go any further, I need to do some calculations. [strong]
  

Although the passive voice has its uses, if it starts to sound officious, distant and dominant, it may be better to change it to the active voice:

• The new rules for working hours are being implemented immediately. [one doing the implementation is not revealed; sounds officious and distant]
  
• We are immediately implementing the new rules for working hours. [less officious and speaks of action; ]
  

Guess conveys tentativeness. What would you think of a CEO who tells analysts:

• Our best guess is that our profit for year’s end will be marginally better than last year’s.

This is weak and makes the CEO sound completely disconnected from the business. It can be reframed more confidently as:

• We expect our profit for the year to be ahead of last year’s.
• Our results should surpass last year’s but this hasn’t been confirmed by the CFO.

The following can be rephrased:

• I guess our debt will be reduced by year’s end. [weak]
  
• Our debt will be reduced by year’s end. [strong]
  

Believe is part of a group of words known as fillers; these reduce conviction when speaking or writing. Like just and feel, they diminish the importance of your statement:

• I believe we have the money available for the project.
• We have the money available for the project.

If you have been asked for your opinion but cannot give an absolutely clear answer, balance your opinion with what is known or what can be done:

• I believe we have the money available for the project but before we do anything I need a couple of hours to check the numbers.
  

Avoid giving your authority to someone else with hope; you have the same rights to the conversation. Instead, say:
 

• I’ve booked the hotel and conference rooms. If there are any changes, please let me know.
  
• See you at the meeting on Friday at 9am.

Avoid not sure as it implies inferiority. It says you have little to no confidence in your abilities.

Rephrase:

• Could you spare half an hour this afternoon?
• I’m not sure about the location. [uncertain without reason]
• We need to research more about the location. [expresses doubt and looks forward]

Avoid sorry and respect yourself and your contribution to any conversation or subject:

• I'm sorry but am I disturbing you? I would like to ask you a couple of questions. [weak]
• I would like to ask you a couple of questions about the Drax report. Would now be a good time?

There is a time and a place for apologising and requiring funds due to no-one’s fault is not one of them.

• I’m sorry but we need extra funds for database development. [weak]
• Due to unforeseen circumstances, we need extra funds for database development. Let me explain …

Probable implies you have not check and you do not know. It says something is likely to happen or be the case but is uncertain:

• Will the report be ready on Thursday? It’s probable. [sounds like you do not care]
• It’s probable the bank will approve the loan. [you do not have an idea]

Eliminate uncertainty where possible:

• I’m not expecting any problems so the report will be ready for Thursday.
• Fred has not confirmed his place on the course as yet. I’ll give him a ring and check.

Be careful with stuff: it is vague; choose a more precise word:  

• These reports are not what we need. [strong]
• Sorry, I can’t make the meeting this afternoon as I have some important stuff to do. [vague]
• Sorry, I can’t make the meeting this afternoon as I have some important reviews to write. [strong]
  

Be careful with things: it can be imprecise; use a more active word: 
 

• We need to do a few things ready for tomorrow’s meeting. [weak]
  
• Before tomorrow’s meeting, we need to check all documents are printed, all participates have their itinerary and each room is set up for each activity. [strong]
• The IT system is slow due to increase internet usage. [strong]

Be careful with get: it can be vague; choose a more accurate verb:  

• We require more funds to complete the project.:  

Informally, get is frequently used but, when writing, make sure the verb balances with the true meaning as get can weaken your comment or request:  

• Can we get these products to the customer by tomorrow? [weak]
• Can we send these products to the customer by tomorrow? [stronger]
• Can we courier these products to the customer by tomorrow? [expresses urgency]

In the first sentence below, get could refer to buy, enquire, investigate or something else:

• We need to get a suitable IT system. [weak]
  
• We need to invest in a suitable IT system. [strong]
  

Be careful with need: it can show dependency and desperateness. Rephrase stating what you need help with:  

• So I can complete this project on time, I estimate a couple of extra people for two weeks is sufficient.  
• We need to run tests on these samples to establish if they are genuine.  

With the right tone, need can be assertive by showing what is essential.

Be careful with suppose: it can show indecision, indifference or that you do not care. If you are not sure, say so and what you can do to solve the issue:  

• This internet connect is unsecure so we shouldn’t send any sensitive data.   
  
• As we don't know if the internet connect is secure, we shouldn't send any sensitive data.
  
• Although there is much to do, everything will be ready for the meeting next week.
  

Be careful with hope: it can suggest that you have no control over a situation.

Rephrase and make the situation clear:  

• Here are the samples you requested.   
  
• I hope the meeting goes well and see you tomorrow. [used to signify encouragement]

Be careful with think: it can reduce the power of your speech and make you look unprofessional or inept.

Replace with a positive verb:

• These products are unsuitable for you because your requirements are different. Therefore, we need to make some changes to better reflect your needs.
• I think it’s a good idea to ring the client tomorrow morning and check they fully understand the figures.