Optimized Qeries Without Using Sub-Correlated Queries

Hi, 
I am used to writing Sub-Correlated queries within my main queries. Although they work fine but i have read alot that they have performance hits. Also, as with time our data has increased, a simple SELECT statement with a few Sub-Queries tends to run slower which may be between 10-15 seconds. Following will be a simple example of what i mostly do:  
SELECT          DISTINCT C.CusID, C.Name, C.Age, 
                        (
                                    SELECT          SUM  (Price)
                                    FROM             CusotmerOrder
                                    WHERE           CusID_fk = CO.CusID_fk
                        )           Total_Order_Price, 
                        (
                                    SELECT          SUM (Concession)
                                    FROM             CusotmerOrder
                                    WHERE           CusID_fk = CO.CusID_fk
                        )           Total_Order_Concession,          
                        (
                                    SELECT          SUM  (Price) - SUM  (Concession)
                                    FROM             CusotmerOrder
                                    WHERE           CusID_fk = CO.CusID_fk                              
                        )           Total_Difference  
FROM             Customer C
INNER JOIN  CustomerOrder CO
ON                  C.CusID = CO.CusID_fk
                        ...... 
WHERE                       (conditions...)  
My question is what would be a better way to handle the above query?  How can i write a better yet simple query with optimized performance. I would also mention that in some of my asp.net applications, i use inline queries assigned to SqlCommand Object. The reason i mention it that since these queries are written in some class files, how would we still accomplish what i have mentioned above. Kindly could any Query Guru guide me writing better queries. I shall be obliged...