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]