I have been receiving Social Security Disability since 1998. Are there any ways that I can increase the amount that I'm already receiving?
Probably not. Social Security disability (SSDI) benefit rates are based on a variable number of a person's highest years of wage-indexed earnings. The number of years used is determined by the age at which the person became disabled.
The only way that your SSDI rate would increase, other than from cost of living increases, is if you have a new year of earnings that's higher than the lowest year previously used to calculate your SSDI rate. But, if you return to work and earn too much, your SSDI benefits could stop altogether. Social Security currently considers earnings of more than $1260 per month to be substantial gainful activity (SGA), and earnings in excess of that amount can cause a person's SSDI benefits to terminate depending on other factors (https://www.ssa.gov/pubs/EN-05-10095.pdf).
Your SSDI benefits will convert to regular Social Security retirement benefits when you reach full retirement age (FRA), and your benefit rate will almost certainly not change as a result. So, the only way you're likely to get more Social Security benefits is if you could qualify for auxiliary or survivor benefits on the record of a parent, spouse or ex-spouse. For example, In order to potentially qualify for benefits on a parent's record, your parent would need to either be drawing benefits or be deceased, and you'd need to be unmarried and your disability would have to have begun prior to age 22.
Social Security also administers a needs based program called Supplemental Security Income (SSI), and if your monthly SSDI rate is currently less than $803 you may want to look into applying for SSI (https://www.ssa.gov/ssi/).