The word "entscheiden" can be used with or without "sich". Both versions are correct, but have different meanings:
Erfolg kann man nicht plannen, oft muss man spontan entscheiden.
You can't plan success, you often must decide spontaneously. The decisions you meet directly concern other persons or other companies. It is you who makes the decisions, but it is not you who is directly influenced by those decisions.
Erfolg kann man nicht plannen, oft muss man SICH spontan entscheiden.
You can't plan success, you often must decide spontaneously for yourself. You make the decisions, and it is you who directly is influenced by them.
The word "anziehen" can be used transitive or reflexive:
Ich ziehe den Pullover an. - I pull on the sweater.
Ich ziehe mich an. - I get myself dressed.
Notice that "mich" is used in accusative which is typical for reflexive usage.
In the sentence "Zur Party ziehe ich heute Abend den neuen Pullover an." you are using the verb in its transitive form. Using it in this way you can be more precise be telling who the person is that gets dressed:
Ich ziehe mir den Pullover an. - I pull on the sweater to myself.
Ich ziehe meinem Sohn den Pullover an. - I pull on the sweater to my son.
Notice that "mir" and "meinem Sohn" stand in dative. So this can not be a reflexive usage!
Because most people pull on cloths to themselves it is clear who gets dressed if you omit the word "mir".